Association of metabolic syndrome with reduced central serotonergic activity

Hypothyroidism and Diabetes-Related Dementia: Focused on Neuronal Dysfunction, Insulin Resistance, and Dyslipidemia

The incidence of dementia is steadily increasing worldwide. The risk factors for dementia are diverse, and include genetic background, environmental factors, sex differences, and vascular abnormalities. Among the subtypes of dementia, diabetes-related dementia is emerging as a complex type of dementia related to metabolic imbalance, due to the increase in the number of patients with metabolic syndrome and dementia worldwide. Thyroid hormones are considered metabolic regulatory hormones and affect various diseases, such as liver failure, obesity, and dementia. Thyroid dysregulation affects various cellular mechanisms and is linked to multiple disease pathologies. In particular, hypothyroidism is considered a critical cause for various neurological problems-such as metabolic disease, depressive symptoms, and dementia-in the central nervous system. Recent studies have demonstrated the relationship between hypothyroidism and brain insulin resistance and dyslipidemia, leading to diabetes-related dementia. Therefore, we reviewed the relationship between hypothyroidism and diabetes-related dementia, with a focus on major features of diabetes-related dementia such as insulin resistance, neuronal dysfunction, and dyslipidemia.

Major depressive disorder: a possible typisation according to serotonin, inflammation, and metabolic syndrome

OBJECTIVE

Major depressive disorder (MDD) is closely related to obesity, inflammation, and insulin resistance, all together being etiologically linked to metabolic syndrome (MetS) development. The depressive disorder has a neuroendocrinological component, co-influencing the MetS, while MetS is characterised by increased cytokine levels, which are known to cause a depressed mood. This study aimed to establish biological subtypes of the depressive disorder based on researched clinical, laboratory, and anthropometric variables.

METHODS

We performed a cross-sectional study on a sample of 293 subjects (145 suffering from a depressive disorder and 148 healthy controls). Results were analysed with multivariate statistical methods as well as with cluster and discriminant analysis. In order to classify depressive disorder on the grounds of laboratory, anthropometric, and clinical parameters, we performed cluster analysis, which resulted in three clusters.

RESULTS

The first cluster is characterised by low platelet serotonin, high cortisol levels, high blood glucose levels, high triglycerides levels, high Hamilton Depression Rating Scale score, high waist circumference, high C-Reactive Protein values, and a high number of previous depressive episodes, was named Combined (Metabolic) depression. The inflammatory depression cluster is defined with average platelet serotonin values, normal cortisol, and all other parameter levels, except for increased IL-6 levels. The serotoninergic depression cluster is characterised by markedly low platelet serotonin, and all other parameters are within the normal range.

CONCLUSIONS

From a biological point of view, depressive disorder is not uniform, and as such, these findings suggest potential clinically useful and generalisable biological subtypes of depressive disorder.

Non-alcoholic Fatty Liver Disease and Depression: Evidence for Genotype × Environment Interaction in Mexican Americans

This study examines the impact of G × E interaction effects on non-alcoholic fatty liver disease (NAFLD) among Mexican Americans in the Rio Grande Valley (RGV) of South Texas. We examined potential G × E interaction using variance components models and likelihood-based statistical inference in the phenotypic expression of NAFLD, including hepatic steatosis and hepatic fibrosis (identified using vibration controlled transient elastography and controlled attenuation parameter measured by the FibroScan Device). We screened for depression using the Beck Depression Inventory-II (BDI-II). We identified significant G × E interactions for hepatic fibrosis × BDI-II. These findings provide evidence that genetic factors interact with depression to influence the expression of hepatic fibrosis.

Pathophysiology linking depression and type 2 diabetes: Psychotherapy, physical exercise, and fecal microbiome transplantation as damage control

Diabetes increases the likelihood of developing depression and vice versa. Research on this bidirectional association has somewhat managed to delineate the interplay among implicated physiological processes. Still, further exploration is required in this context. This review addresses the comorbidity by investigating suspected common pathophysiological mechanisms. One such factor is psychological stress which disturbs the hypothalamic-pituitary-adrenal axis causing hormonal imbalance. This includes elevated cortisol levels, a common biomarker of both depression and diabetes. Disrupted insulin signaling drives the hampered neurotransmission of serotonin, dopamine, and norepinephrine. Also, adipokine hormones such as adiponectin, leptin, and resistin and the orexigenic hormone, ghrelin, are involved in both depression and T2DM. This disarray further interferes with physiological processes encompassing sleep, the gut-brain axis, metabolism, and mood stability. Behavioral coping mechanisms, such as unhealthy eating, mediate disturbed glucose homeostasis, and neuroinflammation. This is intricately linked to oxidative stress, redox imbalance, and mitochondrial dysfunction. However, interventions such as psychotherapy, physical exercise, fecal microbiota transplantation, and insulin-sensitizing agents can help to manage the distressing condition. The possibility of glucagon-like peptide 1 possessing a therapeutic role has also been discussed. Nonetheless, there stands an urgent need for unraveling new correlating targets and biological markers for efficient treatment.

Nonalcoholic Fatty Liver Disease Increases the Risk of Anxiety and Depression

Nonalcoholic fatty liver disease (NAFLD), depression, and anxiety disorders are frequent diseases, and data on mutual influence are inconsistent. The aim of this study was to explore the incidence of depression and anxiety in a large primary care cohort in Germany and to study the impact of NAFLD over a 10-year time frame. Patients with NAFLD diagnosed between 2010 and 2015 were matched to a cohort without NAFLD controlling for age, sex, physician, index year, and Charlson comorbidity index. The primary outcome of the study was the incidence of depression, anxiety, and first prescription of antidepressant drugs. We compared 19,871 patients with NAFLD to 19,871 matched controls. Within 10 years of the index date, 21.2% of patients with NAFLD and 18.2% of controls were diagnosed with depression (P < 0.001). On regression analysis, the hazard ratio (HR) for incidence of depression was 1.21 (P < 0.001). This association was similar for the endpoint of the first prescription of antidepressant drugs (HR, 1.21; P < 0.001). Anxiety disorders were diagnosed in 7.9% of patients with NAFLD and 6.5% of controls during the observation time (P = 0.003). The HR for incidence of anxiety was 1.23 (P < 0.001). This association remained significant in women (P < 0.001), while there was only a trend in men (HR, 1.15; 95% confidence interval, 0.99-1.34; P < 0.067). The risk of developing anxiety disorders was higher in younger patients. Conclusion: NAFLD constitutes an independent risk factor for emerging depression and anxiety even after controlling for confounding comorbidities.

Metformin Promotes Anxiolytic and Antidepressant-Like Responses in Insulin-Resistant Mice by Decreasing Circulating Branched-Chain Amino Acids

Epidemiological studies indicate that insulin resistance (IR), a hallmark of type 2 diabetes, is associated with an increased risk of major depression. Here, we demonstrated that male mice fed a high-fat diet (HFD) exhibited peripheral metabolic impairments reminiscent of IR accompanied by elevated circulating levels of branched-chain amino acids (BCAAs), whereas both parameters were normalized by chronic treatment with metformin (Met). Given the role of BCAAs in the regulation of tryptophan influx into the brain, we then explored the activity of the serotonin (5-HT) system. Our results indicated that HFD-fed mice displayed impairment in the electrical activity of dorsal raphe 5-HT neurons, attenuated hippocampal extracellular 5-HT concentrations and anxiety, one of the most visible and early symptoms of depression. On the contrary, Met stimulated 5-HT neurons excitability and 5-HT neurotransmission while hindering HFD-induced anxiety. Met also promoted antidepressant-like activities as observed with fluoxetine. In light of these data, we designed a modified HFD in which BCAA dietary supply was reduced by half. Deficiency in BCAAs failed to reverse HFD-induced metabolic impairments while producing antidepressant-like activity and enhancing the behavioral response to fluoxetine. Our results suggest that Met may act by decreasing circulating BCAAs levels to favor serotonergic neurotransmission in the hippocampus and promote antidepressant-like effects in mice fed an HFD. These findings also lead us to envision that a diet poor in BCAAs, provided either alone or as add-on therapy to conventional antidepressant drugs, could help to relieve depressive symptoms in patients with metabolic comorbidities.SIGNIFICANCE STATEMENT Insulin resistance in humans is associated with increased risk of anxiodepressive disorders. Such a relationship has been also found in rodents fed a high-fat diet (HFD). To determine whether insulin-sensitizing strategies induce anxiolytic- and/or antidepressant-like activities and to investigate the underlying mechanisms, we tested the effects of metformin, an oral antidiabetic drug, in mice fed an HFD. Metformin reduced levels of circulating branched-chain amino acids, which regulate tryptophan uptake within the brain. Moreover, metformin increased hippocampal serotonergic neurotransmission while promoting anxiolytic- and antidepressant-like effects. Moreover, a diet poor in these amino acids produced similar beneficial behavioral property. Collectively, these results suggest that metformin could be used as add-on therapy to a conventional antidepressant for the comorbidity between metabolic and mental disorders.

How Metabolic State May Regulate Fear: Presence of Metabolic Receptors in the Fear Circuitry

Metabolic status impacts on the emotional brain to induce behavior that maintains energy balance. While hunger suppresses the fear circuitry to promote explorative food-seeking behavior, satiety or obesity may increase fear to prevent unnecessary risk-taking. Here we aimed to unravel which metabolic factors, that transfer information about the acute and the chronic metabolic status, are of primary importance to regulate fear, and to identify their sites of action within fear-related brain regions. We performed a de novo analysis of central and peripheral metabolic factors that can penetrate the blood–brain barrier using genome-wide expression data across the mouse brain from the Allen Brain Atlas (ABA). The central fear circuitry, as defined by subnuclei of the amygdala, the afferent hippocampus, the medial prefrontal cortex and the efferent periaqueductal gray, was enriched with metabolic receptors. Some of their corresponding ligands were known to modulate fear (e.g., estrogen and thyroid hormones) while others had not been associated with fear before (e.g., glucagon, ACTH). Additionally, several of these enriched metabolic receptors were coexpressed with well-described fear-modulating genes (Crh, Crhr1, or Crhr2). Co-expression analysis of monoamine markers and metabolic receptors suggested that monoaminergic nuclei have differential sensitivity to metabolic alterations. Serotonergic neurons expressed a large number of metabolic receptors (e.g., estrogen receptors, fatty acid receptors), suggesting a wide responsivity to metabolic changes. The noradrenergic system seemed to be specifically sensitive to hypocretin/orexin modulation. Taken together, we identified a number of novel metabolic factors (glucagon, ACTH) that have the potential to modulate the fear response. We additionally propose novel cerebral targets for metabolic factors (e.g., thyroid hormones) that modulate fear, but of which the sites of action are (largely) unknown.

Tryptophan depletion under conditions that imitate insulin resistance enhances fatty acid oxidation and induces endothelial dysfunction through reactive oxygen species-dependent and independent pathways

In atherosclerosis-associated pathologic entities characterized by malnutrition and inflammation, L-tryptophan (TRP) levels are low. Insulin resistance is an independent cardiovascular risk factor and induces endothelial dysfunction by increasing fatty acid oxidation. It is also associated with inflammation and low TRP levels. Low TRP levels have been related to worse cardiovascular outcome. This study evaluated the effect of TRP depletion on endothelial dysfunction under conditions that imitate insulin resistance. Fatty acid oxidation, harmful pathways due to increased fatty acid oxidation, and endothelial dysfunction were assessed in primary human aortic endothelial cells cultured under normal glucose, low insulin conditions in the presence or absence of TRP. TRP depletion activated general control non-derepressible 2 kinase and inhibited aryl hydrocarbon receptor. It increased fatty acid oxidation by increasing expression and activity of carnitine palmitoyltransferase 1. Elevated fatty acid oxidation increased the formation of reactive oxygen species (ROS) triggering the polyol and hexosamine pathways, and enhancing protein kinase C activity and methylglyoxal production. TRP absence inhibited nitric oxide synthase activity in a ROS-dependent way, whereas it increased the expression of ICAM-1 and VCAM-1 in a ROS independent and possibly p53-dependent manner. Thus, TRP depletion, an amino acid whose low levels have been related to worse cardiovascular outcome and to inflammatory atherosclerosis-associated pathologic entities, under conditions that imitate insulin resistance enhances fatty acid oxidation and induces endothelial dysfunction through ROS-dependent and independent pathways. These findings may offer new insights at the molecular mechanisms involved in accelerated atherosclerosis that frequently accompanies malnutrition and inflammation.

Lower urinary tract symptoms, benign prostatic hyperplasia and metabolic syndrome

Epidemiological studies have shown that age is the principal unmodifiable risk factor of lower urinary tract symptoms (LUTS). Until the past decade, the process of lower urinary tract ageing was, therefore, considered unmodifiable - as ageing per se. However, the traditional dogma that BPH-related LUTS (BPH-LUTS) is an immutable consequence of old age is no longer acceptable. Results from multiple preclinical and clinical studies indicate that several modifiable, age-related metabolic aberrations (metabolic syndrome, obesity, dyslipidaemia, diabetes) are important determinants in both the development and the progression of BPH-LUTS. Metabolic syndrome and its related comorbidities, such as sex steroid alterations and low-grade inflammation, have been related to BPH-LUTS development and progression. With the correct treatment and recommended lifestyle changes, many individuals with metabolic syndrome might be able to prevent or delay the onset of metabolic-syndrome-related complications; however, whether promoting healthier lifestyles can really alter a man's propensity to develop BPH-LUTS remains to be clarified.

High-fat diet-induced metabolic disorders impairs 5-HT function and anxiety-like behavior in mice

BACKGROUND AND PURPOSE

The link between type 2 diabetes mellitus (T2DM) and depression is bidirectional. However, the possibility that metabolic disorders may elicit anxiogenic-like/depressive-like symptoms or alter the efficacy of antidepressant drugs remains poorly documented. This study explored the influence of T2DM on emotionality and proposed a therapeutic strategy that might be used in depressed diabetic patients.

EXPERIMENTAL APPROACH

Mice were fed a high-fat diet (HFD) and subjected to a full comprehensive metabolic and behavioural analysis to establish correlations between metabolic and psychiatric disorders. In vivo intra-hippocampal microdialysis was also applied to propose a mechanism underpinning the phenotype of mice fed the HFD. Finally, we tested whether chronic administration of the selective 5-HT reuptake inhibitor escitalopram or HFD withdrawal could reverse HFD-induced metabolic and behavioural anomalies.

KEY RESULTS

The increased body weight, hyperglycaemia and impaired glucose tolerance in response to HFD were correlated with anxiogenic-like/depressive-like symptoms. Moreover, this phenotype was associated with decreased extracellular 5-HT levels in the hippocampus which may result from increased sensitivity of the dorsal raphe 5-HT1A autoreceptor. Interestingly, the beneficial effect of prolonged administration of escitalopram was abolished in HFD-fed mice. On the contrary, HFD withdrawal completely reversed metabolic impairments and positively changed symptoms of anxiety, although some behavioural anomalies persisted.

CONCLUSIONS AND IMPLICATIONS

Our data provide clear-cut evidence that both pathologies are finely correlated and associated with impaired 5-HT mediated neurotransmission in the hippocampus. Further experiments are warranted to define the most adequate strategy for the treatment of such co-morbidity.

LINKED ARTICLES

This article is part of a themed section on Updating Neuropathology and Neuropharmacology of Monoaminergic Systems. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v173.13/issuetoc.

Brain signaling systems in the Type 2 diabetes and metabolic syndrome: promising target to treat and prevent these diseases

The changes in the brain signaling systems play an important role in etiology and pathogenesis of Type 2 diabetes mellitus (T2DM) and metabolic syndrome (MS), being a possible cause of these diseases. Therefore, their restoration at the early stages of T2DM and MS can be regarded as a promising way to treat and prevent these diseases and their complications. The data on the functional state of the brain signaling systems regulated by insulin, IGF-1, leptin, dopamine, serotonin, melanocortins and glucagon-like peptide-1, in T2DM and MS, are analyzed. The pharmacological approaches to restoration of these systems and improvement of insulin sensitivity, energy expenditure, lipid metabolism, and to prevent diabetic complications are discussed.

The Effect of Long-Term Intranasal Serotonin Treatment on Metabolic Parameters and Hormonal Signaling in Rats with High-Fat Diet/Low-Dose Streptozotocin-Induced Type 2 Diabetes

In the last years the treatment of type 2 diabetes mellitus (DM2) was carried out using regulators of the brain signaling systems. In DM2 the level of the brain serotonin is reduced. So far, the effect of the increase of the brain serotonin level on DM2-induced metabolic and hormonal abnormalities has been studied scarcely. The present work was undertaken with the aim of filling this gap. DM2 was induced in male rats by 150-day high-fat diet and the treatment with low dose of streptozotocin (25 mg/kg) on the 70th day of experiment. From the 90th day, diabetic rats received for two months intranasal serotonin (IS) at a daily dose of 20 μg/rat. The IS treatment of diabetic rats decreased the body weight, and improved glucose tolerance, insulin-induced glucose utilization, and lipid metabolism. Besides, it restored hormonal regulation of adenylyl cyclase (AC) activity in the hypothalamus and normalized AC stimulation by β-adrenergic agonists in the myocardium. In nondiabetic rats the same treatment induced metabolic and hormonal alterations, some of which were similar to those in DM2 but expressed to a lesser extent. In conclusion, the elevation of the brain serotonin level may be regarded as an effective approach to treat DM2 and its complications.

Elevated alanine aminotransferase independently predicts new onset of depression in employees undergoing health screening examinations

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is the most common cause of elevated alanine aminotransferase (ALT). NAFLD is associated with insulin resistance and hepatic inflammation. Similarly, patients with depression exhibit insulin resistance and increased inflammatory markers. However, no study has shown a clear association between elevated ALT and the development of depression. The aim of the study was to test whether elevated ALT, a surrogate marker for NAFLD, predicts the development of depression.

METHOD

The present prospective cohort study investigated 12 180 employed adults referred for health examinations that included fasting blood tests and anthropometric measurements between 2003 and 2010. Exclusion criteria were: baseline minor/major depression, excessive alcohol consumption and other causes for ALT elevation. Depression was evaluated by the eight-item Patient Health Questionnaire (PHQ-8) score.

RESULTS

The final cohort included 5984 subjects [69.4% men, aged 45.0 (s.d. = 10.24) years]. The incidence rate of minor and major depression was 3.8% and 1.4%, respectively. Elevated ALT was a significant independent predictor for the occurrence of minor [odds ratio (OR) 2.02, 95% confidence interval (CI) 1.40-2.92] and major (OR 3.132, 95% CI 1.81-5.40) depression after adjusting for age, gender, body mass index, education level, serum levels of lipids, glucose, smoking and physical activity. Adding subjective health and affective state parameters (sleep disturbances, self-rated health, anxiety and burnout) as potential mediators only slightly ameliorated the association. Persistently elevated ALT was associated with the greatest risk for minor or major depression as compared with elevation only at baseline or follow-up (p for trend < 0.001).

CONCLUSIONS

Elevated ALT was associated with developing depressive symptoms, thus suggesting that NAFLD may represent an independent modifiable risk factor for depression.

What does irritable bowel syndrome share with non-alcoholic fatty liver disease?

Non-alcoholic fatty liver disease (NAFLD) and irritable bowel syndrome (IBS) are two very common diseases in the general population. To date, there are no studies that highlight a direct link between NAFLD and IBS, but some recent reports have found an interesting correlation between obesity and IBS. A systematic PubMed database search was conducted highlighting that common mechanisms are involved in many of the local and systemic manifestations of NAFLD, leading to an increased cardiovascular risk, and IBS, leading to microbial dysbiosis, impaired intestinal barrier and altered intestinal motility. It is not known when considering local and systemic inflammation/immune system activation, which one has greater importance in NAFLD and IBS pathogenesis. Also, the nervous system is implicated. In fact, inflammation participates in the development of mood disorders, such as anxiety and depression, characteristics of obesity and consequently of NAFLD and, on the other hand, in intestinal hypersensitivity and dysmotility.

Increased inflammation and lower platelet 5-HT in depression with metabolic syndrome

BACKGROUND

Recent studies suggest comorbidity between major depressive disorder (MDD) and metabolic syndrome. For both disorders, impaired serotoninergic neurotransmission and inflammatory factors have been suggested. The objective of this study was to investigate the concentration of platelet serotonin, interleukin-6 (IL-6) and C-reactive protein (CRP) in MDD patients with and without metabolic syndrome. The second goal was to investigate the association of the concentrations of platelet serotonin, IL-6 and CRP with individual components of metabolic syndrome in MDD patients.

METHODS

A total of 145 MDD patients were included in the study (diagnosed according DSM IV TR criteria). The metabolic syndrome was defined according to the criteria of the American National Cholesterol Education Program-Treatment Panel III (ATP III). Inflammation factors (IL-6 and CRP) and platelet serotonin concentration were assessed by the enzyme-linked immunosorbent assay (ELISA).

RESULTS

MDD patients with metabolic syndrome showed lower platelet serotonin and higher IL-6 and CRP concentrations when compared to MDD patients without metabolic syndrome. An inverse correlation was found between platelet serotonin and waist circumference and serum glucose levels. A positive correlation was found between IL-6 and glucose or triglyceride concentrations, while the correlation with HDL cholesterol was negative.

LIMITATIONS

Data on dietary habits or physical activity prior to hospitalisation were not collected. Also, the study was a cross-sectional without a prospective design.

CONCLUSION

Metabolic syndrome in patients with MDD may be associated with reduced concentrations of platelet serotonin and increased concentrations of IL-6 and CRP.