Hippocampal synaptic plasticity and glutamate receptor regulation: influences of diabetes mellitus

Lipin1 ameliorates cognitive ability of diabetic encephalopathy via regulating Ca2+ transfer through mitochondria-associated endoplasmic reticulum membranes

Diabetic encephalopathy (DE) is a common central nervous system complication resulting from diabetes mellitus (DM). While the exact pathogenesis remains unclear, a homeostatic imbalance of mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs) within neurons has been shown to be closely associated with the dysfunctional cognitive pathology of this condition. Our previous work has revealed that phosphatidate phosphatase Lipin1 plays a critical role in the cognitive processes of DE via regulating mitochondrial function. In this study, we reported that the integrity of neuronal MAMs was disrupted in DE mice, which was accompanied by a decrease in the expression of hippocampal Lipin1. With a knock-down of hippocampal Lipin1 in normal mice, ER stress was induced, MAMs structures were impaired and Ca2+ transfer was suppressed. Such effects resulted in mitochondrial dysfunction, synaptic plasticity impairments, and finally cognitive dysfunctions. In contrast, an up-regulation of hippocampal Lipin1 in the DE model partially alleviated these dysfunctions. These results suggest that Lipin1 may ameliorate the cognitive dysfunctions associated with DE via regulating Ca2+ transfers through MAMs. Therefore, targeting Lipin1 may serve as a therapeutic strategy for the clinical treatment of DE.

Association between sleep quality and mild cognitive impairment in Chinese patients with type 2 diabetes mellitus: a cross-sectional study

BACKGROUND

Globally, the number of individuals with type 2 diabetes mellitus (T2DM) is increasing, and they are at a higher risk of developing mild cognitive impairment (MCI) than the general population. Sleep quality is thought to be a modifiable factor that may contribute to MCI, as previous studies have linked it to cognitive function in older adults. However, evidence concerning the association between sleep quality and MCI among patients with T2DM in China is limited. Therefore, this study aims to identify the association between sleep quality and MCI among patients with T2DM in China.

METHODS

This cross-sectional study was conducted among patients with T2DM who were referred to the Endocrinology Department of Xiangya Hospital, Central South University. Data regarding sociodemographic characteristics, lifestyle factors, T2DM-related information, and biochemical indicators were collected. Sleep quality and MCI were evaluated using the Pittsburgh Sleep Quality Index (PSQI) and the Mini-Mental State Examination (MMSE) scale, respectively. The association between sleep quality and MCI was analyzed using univariate and multivariate analyses.

RESULTS

This study included 1,001 patients with T2DM, with a mean age of 60.2 (standard deviation: 10.1) years. Pearson's correlation analysis showed that the total PSQI score was negatively associated with the MMSE score (r=-0.27, P < 0.05). Multivariate analyses based on four models consistently showed that those with higher total PSQI score (aOR = 1.09-1.11, P < 0.05), as well as higher scores on the subjective sleep quality (aOR = 1.32-1.46, P < 0.05), sleep latency (aOR = 1.25-1.32, P < 0.05), sleep duration (aOR = 1.30-1.32, P < 0.05), sleep efficiency (aOR = 1.36-1.41, P < 0.05), sleep disturbance (aOR = 1.66-1.86, P < 0.05), and daily dysfunction (aOR = 1.38-1.48, P < 0.05) were associated with higher rates of MCI.

CONCLUSIONS

Among Chinese patients with T2DM, poor overall sleep quality, subjective sleep quality, sleep latency, sleep duration, sleep efficiency, sleep disturbance, and daily dysfunction were associated with higher rates of MCI. Future studies are needed to examine whether sleep intervention could improve cognitive function in patients with T2DM. It is also suggested for clinicians working with T2DM patients to raise the awareness of cognitive impairment and sleep problems.

Potential compensatory mechanism for cognitive impairment in type 2 diabetes and prediabetes: altered structure-function coupling

Background

Structure-function (SC-FC) coupling may be more sensitive to detecting changes in the brain than any single modality. The aim of this study was to investigate the effects of SC-FC coupling changes on cognition and their interactions in patients with prediabetes and type 2 diabetes mellitus (T2DM).

Methods

A total of 493 participants (119 with normal glucose metabolism (NGM), 125 with prediabetes, and 249 with T2DM) were included in the study. Diffusion-weighted MRI and resting state functional MRI data were used to quantify SC-FC coupling. General linear model and linear regression analysis were used to evaluate the relationship between glucose metabolism, SC-FC coupling, and cognition. Mediation models were used to evaluate the mediating role of regional SC-FC coupling between diabetes-related measures and cognition.

Results

The regional coupling strength of SC-FC varied greatly in different brain regions, but was strongest in the ventral attention and somatmotor network areas. Compared with NGM patients, T2DM patients had higher SC-FC coupling in the default mode network but lower SC-FC coupling in the limbic network. In addition, fasting glucose and HbA1c were associated with weaker SC-FC coupling in the limbic network, fasting insulin with higher SC-FC coupling in the limbic network, and HbA1c with higher SC-FC coupling in the dorsal attention network. Furthermore, through mediated models we found that SC-FC coupling in the limbic network suppressed the association between diabetes-related measures and cognition.

Conclusion

T2DM and diabetes-related measures were associated with abnormal SC-FC coupling of the limbic network. The recombination of SC-FC coupling relationships in the limbic network may indicate a potential compensatory mechanism for cognitive decline that begins in prediabetes.

Efficacy and Safety of Agomelatine in Depressed Patients with Diabetes: A Systematic Review and Meta-Analysis

Major depressive disorder (MDD) and diabetes mellitus (DM) remain among the most prevalent diseases and the most significant challenges faced by medicine in the 21st century. The frequent co-occurrence and bidirectional relationship between the two conditions necessitates the identification of treatment strategies that benefit both. The purpose of this study was to systematically review and meta-analyze data on the efficacy and safety of agomelatine (AGO) in the treatment of patients with depression with comorbid diabetes to explore its potential mechanism of action in both diseases and its impact on diabetic parameters. Following PRISMA guidelines, a total of 11 studies were identified, both preclinical and clinical trials. Agomelatine has shown great potential as a treatment option for patients with diabetes and comorbid depression and anxiety. In addition to improving depressive and anxiety symptoms, it is also beneficial in glycemic control. A meta-analysis demonstrated a statistically significant reduction in glycated hemoglobin (HbA1C) and fasting blood glucose (FBG) levels following AGO administration over a period of 8-16 weeks. The administration of agomelatine was found to result in a significantly greater reduction in HbA1C than that observed with the selective serotonin reuptake inhibitor (SSRI) medications (namely fluoxetine, sertraline, and paroxetine) during 12-16 weeks of therapy. Furthermore, AGO has been found to be at least as effective as SSRIs in reducing depressive symptoms and more effective than SSRIs in reducing anxiety symptoms. The safety of such treatment is similar to SSRIs; no severe adverse events were reported, and the incidence of some side effects, such as insomnia and sexual dysfunction, are even less often reported. Particularly promising is also its potential action in improving some diabetic complications reported in preclinical trials. This might be through mechanisms involving the reduction in oxidative stress, anti-inflammatory effects, and potentially noradrenergic or NMDA receptor modulation. Further clinical studies on larger sample sizes, as well as elucidating its mechanisms of action, especially in the context of diabetic complications, are needed. Research should also focus on identifying the patient subpopulations most likely to benefit from agomelatine treatment.

Cognitive Impairment and Synaptic Dysfunction in Cardiovascular Disorders: The New Frontiers of the Heart-Brain Axis

Within the central nervous system, synaptic plasticity, fundamental to processes like learning and memory, is largely driven by activity-dependent changes in synaptic strength. This plasticity often manifests as long-term potentiation (LTP) and long-term depression (LTD), which are bidirectional modulations of synaptic efficacy. Strong epidemiological and experimental evidence show that the heart-brain axis could be severely compromised by both neurological and cardiovascular disorders. Particularly, cardiovascular disorders, such as heart failure, hypertension, obesity, diabetes and insulin resistance, and arrhythmias, may lead to cognitive impairment, a condition known as cardiogenic dementia. Herein, we review the available knowledge on the synaptic and molecular mechanisms by which cardiogenic dementia may arise and describe how LTP and/or LTD induction and maintenance may be compromised in the CA1 region of the hippocampus by heart failure, metabolic syndrome, and arrhythmias. We also discuss the emerging evidence that endothelial dysfunction may contribute to directly altering hippocampal LTP by impairing the synaptically induced activation of the endothelial nitric oxide synthase. A better understanding of how CV disorders impact on the proper function of central synapses will shed novel light on the molecular underpinnings of cardiogenic dementia, thereby providing a new perspective for more specific pharmacological treatments.

The administration of Cerebrolysin elicits neuroprotective and neurorepair effects in an animal model of type 1 diabetes mellitus

Diabetes mellitus (DM) is a metabolic disorder impacting cerebral function. The administration of Streptozotocin (STZ) is a well-known animal model of insulinopenic type 1 DM in rats. STZ-induced DM results in a myriad of alteration in the periphery and central nervous system (CNS). Cerebrolysin (CBL) is a neuropeptide preparation that promotes synaptic and neuronal plasticity in various animal models. In all cases, CBL was administered when the model was established. This research aims to investigate the neuroprotective and neurorepair effect of CBL on the cytoarchitecture of neurons and spine density in pyramidal neurons of the prefrontal (PFC) and the CA1 region of the dorsal hippocampus, as well as spheroidal neurons of the dentate gyrus (DG), in STZ-induced DM. In the first experimental condition, STZ and CBL are administered at the same time to evaluate the potential preventive effect of CBL. In the second experimental condition, CBL was administered two months after establishing the DM model to measure the potential neurorepair effect of CBL. STZ-induced hyperglycemia remained unaltered by the administration of CBL in both experimental conditions. In the first experimental condition, CBL treatment preserved the neuronal morphology in PFC layer 3, PFC layer 5 and the DG of the hippocampus, while also maintaining spine density in the PFC-3, DG and CA1 hippocampus. Furthermore, CBL induced neurorepair in neurons within the PFC-3, PFC-5 and CA1 regions of the hippocampus, along with an increase in spine density in the PFC-3, DG and CA1 hippocampus. These findings suggest that CBL´s effects on neuroplasticity could be observed before or after the damage was evident.

Association among abnormal glycolipids, reproductive hormones, and cognitive dysfunction in female patients with bipolar disorder

BACKGROUND

Patients with bipolar disorder (BD) show abnormalities in glucolipid metabolism and reproductive hormone levels, which are of concern in women with BD. This study was dedicated to investigating the glucolipid and reproductive hormone levels of female patients, and to preliminarily investigating their relationships with cognition.

METHODS

A total of 58 unmedicated female BD patients, 61 stable-medicated female BD patients, and 63 healthy controls (HC) were recruited in this study. Serum glycolipid indexes and reproductive hormones were measured. Cognitive function was assessed using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS) and the Stroop Color-Word Test (Stroop test).

RESULTS

Patients with BD showed significant cognitive impairment (p < 0.05), which was not affected by medication. Triglycerides (TG), luteinizing hormone (LH), and high-density lipoprotein cholesterol (HDL-c) were altered in stable-medicated BD patients. In addition, regression analysis showed that progesterone (PRGE) and prolactin (PRL) were negatively associated with cognitive performance in stable-medicated BD patients.

CONCLUSIONS

Female BD patients may have cognitive deficits and abnormal levels of glycolipids and reproductive hormones. And abnormal levels of glycolipids and reproductive hormones may be associated with cognitive dysfunction in female BD patients.

Temporal and region-specific tau hyperphosphorylation in the medulla and forebrain coincides with development of functional changes in male obese Zucker rats

Metabolic syndrome (MetS) is associated with development of tauopathies that contribute to cognitive decline. Without functional leptin receptors, male obese Zucker rats (OZRs) develop MetS, and they have increased phosphorylated tau (ptau) with impaired cognitive function. In addition to regulating energy balance, leptin enhances activation of the hippocampus, which is essential for spatial learning and memory. Whether spatial learning and memory are always impaired in OZRs or develop with MetS is unknown. We hypothesized that male OZRs develop MetS traits that promote regional increases in ptau and functional deficits associated with those brain regions. In the medulla and cortex, tau-pSer199,202 and tau-pSer396 were comparable in juvenile (7-8 wk old) lean Zucker rats (LZRs) and OZRs but increased in 18- to 19-wk-old OZRs. Elevated tau-pSer396 was concentrated in the dorsal vagal complex of the medulla, and by this age OZRs had hypertension with increased arterial pressure variability. In the hippocampus, tau-pSer199,202 and tau-pSer396 were still comparable in 18- to 19-wk-old OZRs and LZRs but elevated in 28- to 29-wk-old OZRs, with emergence of deficits in Morris water maze performance. Comparable escape latencies observed during acquisition in 18- to 19-wk-old OZRs and LZRs were increased in 28- to 29-wk-old OZRs, with greater use of nonspatial search strategies. Increased ptau developed with changes in the insulin/phosphatidylinositol 3-kinase (PI3K)/Akt signaling pathway in the hippocampus and cortex but not medulla, suggesting different underlying mechanisms. These data demonstrate that leptin is not required for spatial learning and memory in male OZRs. Furthermore, early development of MetS-associated autonomic dysfunction by the medulla may be predictive of later hippocampal dysfunction and cognitive impairment.NEW & NOTEWORTHY Male obese Zucker rats (OZRs) lack functional leptin receptors and develop metabolic syndrome (MetS). At 16-19 wk, OZRs are insulin resistant, with increased ptau in dorsal medulla and impaired autonomic regulation of AP. At 28-29 wk OZRs develop increased ptau in hippocampus with deficits in spatial learning and memory. Juvenile OZRs lack elevated ptau and these deficits, demonstrating that leptin is not essential for normal function. Elevated ptau and deficits emerge before the onset of diabetes in insulin-resistant OZRs.

Aerobic exercise improves cognitive impairment in mice with type 2 diabetes by regulating the MALAT1/miR-382-3p/BDNF signaling pathway in serum-exosomes

BACKGROUND

It has been documented that aerobic exercise (AE) has a positive effect on improving cognitive function in type 2 diabetes (T2DM) patients. Here, we tried to explore how AE regulates the expression of long non-coding RNA in serum-exosomes (Exos), thereby affecting cognitive impairment in T2DM mice as well as its potential molecular mechanism.

METHODS

T2DM mouse models were constructed, and serum-Exos were isolated for whole transcriptome sequencing to screen differentially expressed lncRNA and mRNA, followed by prediction of downstream target genes. The binding ability of miR-382-3p with a long non-coding RNA MALAT1 and brain-derived neurotrophic factor (BDNF) was explored. Then, primary mouse hippocampal neurons were collected for in vitro mechanism verification, as evidenced by the detection of hippocampal neurons' vitality, proliferation, and apoptosis capabilities, and insulin resistance. Finally, in vivo mechanism verification was performed to assess the effect of AE on insulin resistance and cognitive disorder.

RESULTS

Transcriptome sequencing analysis showed that MALAT1 was lowly expressed and miR-382-3p was highly expressed in serum-Exos samples of T2DM mice. There were targeted binding sites between MALAT1 and miR-382-3p and between miR-382-3p and BDNF. In vitro experiments showed that MALAT1 upregulated BDNF expression by inhibiting miR-382-3p. Silencing MALAT1 or overexpressing miR-382-3p could reduce the expression of INSR, IRS-1, IRS-2, PI3K/AKT, and Ras/MAPK, inhibit neuronal proliferation, and promote apoptosis. In vivo experiments further confirmed that AE could increase the expression of MALAT1 in serum-Exos to competitively inhibit miR-382-3p and upregulate BDNF expression, thereby improving cognitive impairment in T2DM mice.

CONCLUSION

AE may upregulate the expression of MALAT1 in serum-Exos to competitively inhibit miR-382-3p and upregulate BDNF expression, thus improving cognitive impairment in T2DM mice.

Molecular mechanisms underlying hyperglycemia associated cognitive decline

Diabetes mellitus (DM) is a metabolic disease characterized by chronic hyperglycemia. DM can lead to a number of secondary complications affecting multiple organs in the body including the eyes, kidney, heart, and brain. The most common effect of hyperglycemia on the brain is cognitive decline. It has been estimated that 20-70% of people with DM have cognitive deficits. High blood sugar affects key brain areas involved in learning, memory, and spatial navigation, and the structural complexity of the brain has made it prone to a variety of pathological disorders, including T2DM. Studies have reported that cognitive decline can occur in people with diabetes, which could go undetected for several years. Moreover, studies on brain imaging suggest extensive effects on different brain regions in patients with T2D. It remains unclear whether diabetes-associated cognitive decline is a consequence of hyperglycemia or a complication that co-occurs with T2D. The exact mechanism underlying cognitive impairment in diabetes is complex; however, impaired glucose metabolism and abnormal insulin function are thought to play important roles. In this review, we have tried to summarize the effect of hyperglycemia on the brain structure and functions, along with the potential mechanisms underlying T2DM-associated cognitive decline.

Investigating the effects of Citrullus colocynthis on cognitive performance and anxiety-like behaviors in STZ-induced diabetic rats

Background: Diabetes can impair cognitive performance and lead to dementia. Patients with type 1 diabetes mellitus (T1DM) are reported with different levels of cognitive dysfunctions in various cognitive domains ranging from general intellectual testing to specific deficits with visuospatial abilities, motor speed, writing, attention, reading, and psychomotor efficiency. The present study aimed to investigate the effect of Citrullus colocynthis on cognitive functions.Methods: A total of 42 male Wistar rats (3-4  months old and weighing 200-250  g) were tested in the current study. Rats were randomly allocated into 3 groups of control, Diabetes, and Diabetes + Drug. The diabetic rats received Citrullus colocynthis extraction orally. The behavioral tests included the open field, elevated plus maze (EPM), novel object recognition (NOR), passive avoidance tests, and Morris Water Maze (MWM) tests. Data were analyzed using student and paired t-tests via SPSS software version 16.Results: Our results showed the protective effects of Citrullus colocynthis administration against cognitive impairments. This is followed by STZ-induced diabetes in the MWM, novel object recognition, and passive avoidance tasks. Also, it was found that Citrullus colocynthis improved anxiety in diabetic rats.Conclusion According to the findings of this study, the administration of 200  mg/kg C. colocynthis once per day for 40  days can lead to ameliorated cognitive impairments and antidiabetic effects such as increasing body weight and decreasing FBS.

Metabolic perspective of astrocyte dysfunction in Alzheimer's disease and type 2 diabetes brains

The term type III diabetes (T3DM) has been proposed for Alzheimer's disease (AD) due to the shared molecular and cellular features between type 2 diabetes (T2DM) and insulin resistance-associated memory deficits and cognitive decline in elderly individuals. Astrocytes elicit neuroprotective or deleterious effects in AD progression and severity. Patients with T2DM are at a high risk of cognitive impairment, and targeting astrocytes might be promising in alleviating neurodegeneration in the diabetic brain. Recent studies focusing on cell-specific activities in the brain have revealed the important role of astrocytes in brain metabolism (e.g., glucose metabolism, lipid metabolism), neurovascular coupling, synapses, and synaptic plasticity. In this review, we discuss how astrocytes and their dysfunction result in multiple pathological and clinical features of AD and T2DM from a metabolic perspective and the potential comorbid mechanism in these two diseases from the perspective of astrocytes.

The Role of Glucagon-Like Peptide-1 Receptor Agonists (GLP-1 RA) in Diabetes-Related Neurodegenerative Diseases

Recent clinical guidelines have emphasized the importance of screening for cognitive impairment in older adults with diabetes, however, there is still a lack of understanding about the drug therapy. Glucagon-like peptide 1 receptor agonists (GLP-1 RAs) are widely used in the treatment of type 2 diabetes and potential applications may include the treatment of obesity as well as the adjunctive treatment of type 1 diabetes mellitus in combination with insulin. Growing evidence suggests that GLP-1 RA has the potential to treat neurodegenerative diseases, particularly in diabetes-related Alzheimer’s disease (AD) and Parkinson’s disease (PD). Here, we review the molecular mechanisms of the neuroprotective effects of GLP-1 RA in diabetes-related degenerative diseases, including AD and PD, and their potential effects.

Receptor for advanced glycation end products aggravates cognitive deficits in type 2 diabetes through binding of C-terminal AAs 2-5 to mitogen-activated protein kinase kinase 3 (MKK3) and facilitation of MEKK3-MKK3-p38 module assembly

In this study, we explored the precise mechanisms underlying the receptor for advanced glycation end products (RAGE)‐mediated neuronal loss and behavioral dysfunction induced by hyperglycemia. We used immunoprecipitation (IP) and GST pull‐down assays to assess the interaction between RAGE and mitogen‐activated protein kinase kinase 3 (MKK3). Then, we investigated the effect of specific mutation of RAGE on plasticity at hippocampal synapses and behavioral deficits in db/db mice through electrophysiological recordings, morphological assays, and behavioral tests. We discovered that RAGE binds MKK3 and that this binding is required for assembly of the MEKK3‐MKK3‐p38 signaling module. Mechanistically, we found that activation of p38 mitogen‐activated protein kinase (MAPK)/NF‐κB signaling depends on mediation of the RAGE‐MKK3 interaction by C‐terminal RAGE (ctRAGE) amino acids (AAs) 2‐5. We found that ctRAGE R2A‐K3A‐R4A‐Q5A mutation suppressed neuronal damage, improved synaptic plasticity, and alleviated behavioral deficits in diabetic mice by disrupting the RAGE‐MKK3 conjugation. High glucose induces direct binding of RAGE and MKK3 via ctRAGE AAs 2‐5, which leads to assembly of the MEKK3‐MKK3‐p38 signaling module and subsequent activation of the p38MAPK/NF‐κB pathway, and ultimately results in diabetic encephalopathy (DE).

Altered Structural and Functional MRI Connectivity in Type 2 Diabetes Mellitus Related Cognitive Impairment: A Review

Type 2 diabetes mellitus (T2DM) is associated with cognitive impairment in many domains. There are several pieces of evidence that changes in neuronal neuropathies and metabolism have been observed in T2DM. Structural and functional MRI shows that abnormal connections and synchronization occur in T2DM brain circuits and related networks. Neuroplasticity and energy metabolism appear to be principal effector systems, which may be related to amyloid beta (Aβ) deposition, although there is no unified explanation that includes the complex etiology of T2DM with cognitive impairment. Herein, we assume that cognitive impairment in diabetes may lead to abnormalities in neuroplasticity and energy metabolism in the brain, and those reflected to MRI structural connectivity and functional connectivity, respectively.

Concussion/Mild Traumatic Brain Injury (TBI) Induces Brain Insulin Resistance: A Positron Emission Tomography (PET) Scanning Study

Brain injury/concussion is a growing epidemic throughout the world. Although evidence supports association between traumatic brain injury (TBI) and disturbance in brain glucose metabolism, the underlying molecular mechanisms are not well established. Previously, we reported the release of cellular prion protein (PrPc) from the brain to circulation following TBI. The PrPc level was also found to be decreased in insulin-resistant rat brains. In the present study, we investigated the molecular link between PrPc and brain insulin resistance in a single and repeated mild TBI-induced mouse model. Mild TBI was induced in mice by dropping a weight (~95 g at 1 m high) on the right side of the head. The procedure was performed once and thrice (once daily) for single (SI) and repeated induction (RI), respectively. Micro PET/CT imaging revealed that RI mice showed significant reduction in cortical, hippocampal and cerebellum glucose uptake compared to SI and control. Mice that received RI also showed significant motor and cognitive deficits. In co-immunoprecipitation, the interaction between PrPc, flotillin and Cbl-associated protein (CAP) observed in the control mice brains was disrupted by RI. Lipid raft isolation showed decreased levels of PrPc, flotillin and CAP in the RI mice brains. Based on observation, it is clear that PrPc has an interaction with CAP and the dislodgment of PrPc from cell membranes may lead to brain insulin resistance in a mild TBI mouse model. The present study generated a new insight into the pathogenesis of brain injury, which may result in the development of novel therapy.

Intranasal Insulin for Alzheimer's Disease

Brain insulin signaling contributes to memory function and might be a viable target in the prevention and treatment of memory impairments including Alzheimer's disease. This short narrative review explores the potential of central nervous system (CNS) insulin administration via the intranasal pathway to improve memory performance in health and disease, with a focus on the most recent results. Proof-of-concept studies and (pilot) clinical trials in individuals with mild cognitive impairment or Alzheimer's disease indicate that acute and prolonged intranasal insulin administration enhances memory performance, and suggest that brain insulin resistance is a pathophysiological factor in Alzheimer's disease with or without concomitant metabolic dysfunction. Intranasally administered insulin is assumed to trigger improvements in synaptic plasticity and regional glucose uptake as well as alleviations of Alzheimer's disease neuropathology; additional contributions of changes in hypothalamus-pituitary-adrenocortical axis activity and sleep-related mechanisms are discussed. While intranasal insulin delivery has been conclusively demonstrated to be effective and safe, the recent outcomes of large-scale clinical studies underline the need for further investigations, which might also yield new insights into sex differences in the response to intranasal insulin and contribute to the optimization of delivery devices to grasp the full potential of intranasal insulin for Alzheimer's disease.

Exploring brain insulin resistance in adults with bipolar depression using extracellular vesicles of neuronal origin

Accumulating evidence suggests that disrupted insulin signaling is involved in bipolar disorder (BD) pathogenesis. Herein, we aimed to directly explore the potential role of neuronal insulin signaling using an innovative technique based on biomarkers derived from plasma extracellular vesicles enriched for neuronal origin (NEVs). We leveraged plasma samples from a randomized, double-blind, placebo-controlled, 12-week clinical trial evaluating infliximab as a treatment of bipolar depression. We isolated NEVs using immunoprecipitation against neuronal marker L1CAM from samples collected at baseline and weeks 2, 6 and 12 (endpoint) and measured NEV biomarkers using immunoassays. We assessed neuronal insulin signaling at its first node (IRS-1) and along the canonical (Akt, GSK-3β, p70S6K) and alternative (ERK1/2, JNK and p38-MAPK) pathways. A subset of participants (n = 27) also underwent whole-brain magnetic resonance imaging (MRI) at baseline and endpoint. Pre-treatment, NEV biomarkers of insulin signaling were independently associated with cognitive function and MRI measures (i.e. hippocampal and ventromedial prefrontal cortex [vmPFC] volumes). In fact, the association between IRS-1 phosphorylation at serine site 312 (pS312-IRS-1), an indicator of insulin resistance, and cognitive dysfunction was mediated by vmPFC volume. In the longitudinal analysis, patients treated with infliximab, a tumor necrosis factor-alpha antagonist with known insulin sensitizing properties, compared to those treated with placebo, had augmented phosphorylation of proteins from the alternative pathway. Infliximab responders had significant increases in phosphorylated JNK levels, relative to infliximab non-responders and placebo responders. In addition, treatment with infliximab resulted in increase in MRI measures of brain volume; treatment-related changes in the dorsolateral prefrontal cortex volume were mediated by changes in biomarkers from the insulin alternative pathway. In conclusion, our findings support the idea that brain insulin signaling is a target for further mechanistic and therapeutic investigations.

Hyperglycemia induces RAGE-dependent hippocampal spatial memory impairments

Diabetes is a prevalent metabolic disorder that has long been associated with changes in different regions of the brain, including the hippocampus. Changes in hippocampal synaptic plasticity and subsequent impairment in cognitive functions such as learning and memory, are well documented in animal models of type 1 and type 2 diabetes. It is known that RAGE contributes to peripheral micro- and macro-vascular complications of diabetes. However, it is still unknown if RAGE plays a similar role in the development of CNS complications of diabetes. Therefore, we hypothesize that RAGE contributes to cognitive dysfunction, such as learning and memory impairments, in a mouse model of STZ-induced hyperglycemia. Control and STZ-induced hyperglycemic mice from WT and RAGE-KO groups were used for the behavioral experiments. While STZ-induced hyperglycemia decreased locomotor activity in the open field (OF) test, it did not affect the recognition memory in the novel object recognition (NOR) test in either genotype. Spatial memory, however, was impaired in STZ-induced hyperglycemic mice in WT but not in RAGE-KO group in both the Barnes maze (BM) and the Morris water maze (MWM) tests. Consistently, the RAGE antagonist FPS-ZM1 protected WT STZ-induced hyperglycemic mice from spatial memory impairment in the BM test. Our findings indicate that the parameters associated with locomotor activity and recognition memory were independent of RAGE in STZ-induced hyperglycemic mice. In contrast, the parameters associated with hippocampal-dependent spatial memory were dependent on RAGE expression.

RAGE signaling is required for AMPA receptor dysfunction in the hippocampus of hyperglycemic mice

Diabetes in humans has been associated for a long time with cognitive dysfunction. In rodent animal models, cognitive dysfunction can manifest as impaired hippocampal synaptic plasticity. Particular attention has been concentrated on the receptor for advanced glycation end products (RAGE), which is implicated in multiple diabetic complications involving the development of vascular and peripheral nerve abnormalities. In this study, we hypothesize that RAGE signaling alters glutamate receptor function and expression, impairing synaptic transmission in the hippocampus. Using preparations of hippocampal slices from male mice, we show a RAGE-dependent decrease in long-term potentiation (LTP) and an increase in paired-pulse facilitation (PPF) following streptozotocin (STZ)-induced diabetes. Consistently, in hippocampal cultures from male and female neonatal mice, high glucose caused a RAGE-dependent reduction of AMPA- but not NMDA-evoked currents, and an increase in cytosolic reactive oxygen species (ROS). Consistently, when cultures were co-treated with high glucose and the RAGE antagonist FPS-ZM1, AMPA-evoked currents were unchanged. Hippocampi from STZ-induced hyperglycemic wild type (WT) mice showed increased RAGE expression concomitant with a decrease of both expression and phosphorylation (Ser 831 and 845) of the AMPA GluA1 subunit. We found these changes correlated to activation of the MAPK pathway, consistent with decreased pJNK/JNK ratio and the JNK kinase, pMEK7. As no changes in expression or phosphorylation of regulatory proteins were observed in hippocampi from STZ-induced hyperglycemic RAGE-KO mice, we report a RAGE-dependent impairment in the hippocampi of hyperglycemic WT mice, with reduced AMPA receptor expression/function and LTP deficits.

Alterations in effective connectivity within the Papez circuit are correlated with insulin resistance in T2DM patients without mild cognitive impairment

Insulin resistance (IR) can significantly affect the hippocampus, a component of a larger memory circuit called the Papez circuit. This study was performed to identify altered effective connectivity within the Papez circuit in type 2 diabetes mellitus (T2DM) patients without mild cognitive impairment (MCI) and to determine the relationships between these alterations and IR. T2DM patients without MCI (n = 105) and age-, sex-, and education-matched healthy controls (n = 106) were included in this study. Granger causality analysis (GCA) with seed regions in the hippocampus was performed to identify abnormal effective connectivity in the brains of T2DM patients without MCI. Furthermore, correlation analysis was conducted to detect the association between aberrant effective connectivity and IR in T2DM patients without MCI. Compared to healthy controls, T2DM patients without MCI demonstrated abnormal directional connectivity both to and from the hippocampus; the main abnormalities were located in several brain areas, including the cingulate cortex, amygdala, and prefrontal cortex, all of which are components of the Papez circuit. This altered effective connectivity network in the Papez circuit was correlated with IR in T2DM patients without MCI. Effective connectivity network alterations within the Papez circuit occurred prior to the appearance of mild cognitive deficits in T2DM patients and were correlated with IR. The current study may improve our understanding of the IR-related neurological mechanisms involved in T2DM.

Acute Hyperglycemia Increases Brain Pregenual Anterior Cingulate Cortex Glutamate Concentrations in Type 1 Diabetes

The brain mechanisms underlying the association of hyperglycemia with depressive symptoms are unknown. We hypothesized that disrupted glutamate metabolism in pregenual anterior cingulate cortex (ACC) in type 1 diabetes (T1D) without depression affects emotional processing. Using proton MRS, we measured glutamate concentrations in ACC and occipital lobe cortex (OCC) in 13 subjects with T1D without major depression (HbA_1c 7.1 ± 0.7% [54 ± 7 mmol/mol]) and 11 healthy control subjects without diabetes (HbA_1c 5.5 ± 0.2% [37 ± 3 mmol/mol]) during fasting euglycemia followed by a 60-min +5.5 mmol/L hyperglycemic clamp (HG). Intrinsic neuronal activity was assessed using resting-state blood oxygen level-dependent functional MRI to measure the fractional amplitude of low-frequency fluctuations in slow-4 band (fALFF4). Emotional processing and depressive symptoms were assessed using emotional tasks (emotional Stroop task, self-referent encoding task [SRET]) and clinical ratings (Hamilton Depression Rating Scale [HAM-D], Symptom Checklist-90 Revised [SCL-90-R]), respectively. During HG, ACC glutamate increased (1.2 mmol/kg, 10% P = 0.014) while ACC fALFF4 was unchanged (-0.007, -2%, P = 0.449) in the T1D group; in contrast, glutamate was unchanged (-0.2 mmol/kg, -2%, P = 0.578) while fALFF4 decreased (-0.05, -13%, P = 0.002) in the control group. OCC glutamate and fALFF4 were unchanged in both groups. T1D had longer SRET negative word response times (P = 0.017) and higher depression rating scores (HAM-D P = 0.020, SCL-90-R depression P = 0.008). Higher glutamate change tended to associate with longer emotional Stroop response times in T1D only. Brain glutamate must be tightly controlled during hyperglycemia because of the risk for neurotoxicity with excessive levels. Results suggest that ACC glutamate control mechanisms are disrupted in T1D, which affects glutamatergic neurotransmission related to emotional or cognitive processing. Increased prefrontal glutamate during acute hyperglycemic episodes could explain our previous findings of associations among chronic hyperglycemia, cortical thinning, and depressive symptoms in T1D.

Nicotinamide Mononucleotide Administration Prevents Experimental Diabetes-Induced Cognitive Impairment and Loss of Hippocampal Neurons

Diabetes predisposes to cognitive decline leading to dementia and is associated with decreased brain NAD⁺ levels. This has triggered an intense interest in boosting nicotinamide adenine dinucleotide (NAD⁺) levels to prevent dementia. We tested if the administration of the precursor of NAD⁺, nicotinamide mononucleotide (NMN), can prevent diabetes-induced memory deficits. Diabetes was induced in Sprague-Dawley rats by the administration of streptozotocin (STZ). After 3 months of diabetes, hippocampal NAD⁺ levels were decreased (p = 0.011). In vivo localized high-resolution proton magnetic resonance spectroscopy (MRS) of the hippocampus showed an increase in the levels of glucose (p < 0.001), glutamate (p < 0.001), gamma aminobutyric acid (p = 0.018), myo-inositol (p = 0.018), and taurine (p < 0.001) and decreased levels of N-acetyl aspartate (p = 0.002) and glutathione (p < 0.001). There was a significant decrease in hippocampal CA1 neuronal volume (p < 0.001) and neuronal number (p < 0.001) in the Diabetic rats. Diabetic rats showed hippocampal related memory deficits. Intraperitoneal NMN (100 mg/kg) was given after induction and confirmation of diabetes and was provided on alternate days for 3 months. NMN increased brain NAD⁺ levels, normalized the levels of glutamate, taurine, N-acetyl aspartate (NAA), and glutathione. NMN-treatment prevented the loss of CA1 neurons and rescued the memory deficits despite having no significant effect on hyperglycemic or lipidemic control. In hippocampal protein extracts from Diabetic rats, SIRT1 and PGC-1α protein levels were decreased, and acetylation of proteins increased. NMN treatment prevented the diabetes-induced decrease in both SIRT1 and PGC-1α and promoted deacetylation of proteins. Our results indicate that NMN increased brain NAD⁺, activated the SIRT1 pathway, preserved mitochondrial oxidative phosphorylation (OXPHOS) function, prevented neuronal loss, and preserved cognition in Diabetic rats.

Impact of exercise on hippocampal neurogenesis in hyperglycemic diabetes

Hyperglycemic diabetes is a chronic metabolic disorder characterized by high level of plasma glucose. Numerous studies have shown that hy-perglycemic diabetes leads to brain dysfunction including cognitive im-pairment and emotional disorders. This study evaluated the impact of exercise on brain dysfunction, hippocampal neurogenesis, and cogni-tive impairment in hyperglycemic diabetes. The present study suggests that exercise improves hyperglycemic control and prevents decline of cognition through increasing hippocampal neurogenesis. Understanding the mechanism of exercise for hippocampal neurogenesis can lead to the development of therapeutic strategies for metabolic disorders.

Sustained Cerebrovascular and Cognitive Benefits of Resveratrol in Postmenopausal Women

Deficits in the cerebral microcirculation contribute to age-related cognitive decline. In a pilot study of postmenopausal women, we found that supplementation with a low dose of resveratrol, a phytoestrogen, for 14 weeks improved cerebrovascular and cognitive functions. We have since undertaken a larger, longer term study to confirm these benefits. Postmenopausal women aged 45-85 years (n = 129) were randomized to take placebo or 75 mg trans-resveratrol twice daily for 12 months. Effects on cognition, cerebral blood flow, cerebrovascular responsiveness (CVR) and cardiometabolic markers (blood pressure, diabetes markers and fasting lipids) were assessed. Compared to placebo, resveratrol improved overall cognitive performance (P < 0.001) and attenuated the decline in CVR to cognitive stimuli (P = 0.038). The latter effect was associated with reduction of fasting blood glucose (r = -0.339, P = 0.023). This long-term study confirms that regular consumption of resveratrol can enhance cognitive and cerebrovascular functions in postmenopausal women, with the potential to slow cognitive decline due to ageing and menopause.

Brain Metabolism Alterations in Type 2 Diabetes: What Did We Learn From Diet-Induced Diabetes Models?

Type 2 diabetes (T2D) is a metabolic disease with impact on brain function through mechanisms that include glucose toxicity, vascular damage and blood–brain barrier (BBB) impairments, mitochondrial dysfunction, oxidative stress, brain insulin resistance, synaptic failure, neuroinflammation, and gliosis. Rodent models have been developed for investigating T2D, and have contributed to our understanding of mechanisms involved in T2D-induced brain dysfunction. Namely, mice or rats exposed to diabetogenic diets that are rich in fat and/or sugar have been widely used since they develop memory impairment, especially in tasks that depend on hippocampal processing. Here we summarize main findings on brain energy metabolism alterations underlying dysfunction of neuronal and glial cells promoted by diet-induced metabolic syndrome that progresses to a T2D phenotype.

Liraglutide Suppresses Tau Hyperphosphorylation, Amyloid Beta Accumulation through Regulating Neuronal Insulin Signaling and BACE-1 Activity

Neuronal insulin resistance is a significant feature of Alzheimer's disease (AD). Accumulated evidence has revealed the possible neuroprotective mechanisms of antidiabetic drugs in AD. Liraglutide, a glucagon-like peptide-1 (GLP-1) analog and an antidiabetic agent, has a benefit in improving a peripheral insulin resistance. However, the neuronal effect of liraglutide on the model of neuronal insulin resistance with Alzheimer's formation has not been thoroughly investigated. The present study discovered that liraglutide alleviated neuronal insulin resistance and reduced beta-amyloid formation and tau hyperphosphorylation in a human neuroblostoma cell line, SH-SY5Y. Liraglutide could effectively reverse deleterious effects of insulin overstimulation. In particular, the drug reversed the phosphorylation status of insulin receptors and its major downstream signaling molecules including insulin receptor substrate 1 (IRS-1), protein kinase B (AKT), and glycogen synthase kinase 3 beta (GSK-3β). Moreover, liraglutide reduced the activity of beta secretase 1 (BACE-1) enzyme, which then decreased the formation of beta-amyloid in insulin-resistant cells. This indicated that liraglutide can reverse the defect of phosphorylation status of insulin signal transduction but also inhibit the formation of pathogenic Alzheimer's proteins like Aβ in neuronal cells. We herein provided the possibility that the liraglutide-based therapy may be able to reduce such deleterious effects caused by insulin resistance. In view of the beneficial effects of liraglutide administration, these findings suggest that the use of liraglutide may be a promising therapy for AD with insulin-resistant condition.

Protective effect of hydrogen sulfide on oxidative stress-induced neurodegenerative diseases

Hydrogen sulfide is an antioxidant molecule that has a wide range of biological effects against oxidative stress. Balanced oxidative stress is also vital for maintaining cellular function in biological system, where reactive oxygen species are the main source of oxidative stress. When the normal redox balance is disturbed, deoxyribonucleic acid, lipid, and protein molecules are oxidized under pathological conditions, like diabetes mellitus that leads to diabetic peripheral neuropathy. In diabetes mellitus-induced diabetic peripheral neuropathy, due to hyperglycemia, pancreatic beta cell (β cell) shows resistance to insulin secretion. As a consequence, glucose metabolism is disturbed in neuronal cells which are distracted from providing proper cell signaling pathway. Not only diabetic peripheral neuropathy but also other central damages occur in brain neuropathy. Neurological studies regarding type 1 diabetes mellitus patients with Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis have shown changes in the central nervous system because high blood glucose levels (HbA1c) appeared with poor cognitive function. Oxidative stress plays a role in inhibiting insulin signaling that is necessary for brain function. Hydrogen sulfide exhibits antioxidant effects against oxidative stress, where cystathionine β synthase, cystathionine γ lyase, and 3-mercaptopyruvate sulfurtransferase are the endogenous sources of hydrogen sulfide. This review is to explore the pathogenesis of diabetes mellitus-induced diabetic peripheral neuropathy and other neurological comorbid disorders under the oxidative stress condition and the anti-oxidative effects of hydrogen sulfide.

Diabetes and the link between neuroplasticity and glutamate in the aging human motor cortex

OBJECTIVES

In older adults, type-2 diabetes mellitus (T2DM) impacts cognition and increases dementia risk. Prior studies suggest that impaired neuroplasticity may contribute to the cognitive decline in T2DM, but the underlying mechanisms of altered neuroplasticity are unclear. We investigated the relationship of the concentration of glutamatergic metabolites with measures of cortical plasticity in older adults across the spectrum of glucose intolerance/insulin resistance.

METHODS

Forty adults (50-87 years: 17-T2DM, 14-pre-diabetes, 9-controls) underwent magnetic resonance spectroscopy to quantify glutamate and other key metabolites within a 2 cm3 region around the hand knob of the left primary motor cortex. Thirty-six also underwent a separate transcranial magnetic stimulation (TMS) assessment of cortical excitability and plasticity using single-pulse TMS and intermittent theta-burst stimulation targeting the same brain region.

RESULTS

Group differences were observed in relative concentrations of glutamine (p = .028), glucose (p = .008), total cholines (p = .048), and the glutamine/glutamate ratio (p = .024). Cortical plasticity was reduced in both T2DM and pre-diabetes groups relative to controls (p-values < .05). Only the T2DM group showed a significant positive association between glutamate concentration and plasticity (r = .56, p = .030).

CONCLUSIONS

Neuroplastic mechanisms are already impaired in pre-diabetes. In T2DM, reduced cortico-motor plasticity is associated with lower cortical glutamate concentration.

SIGNIFICANCE

Impaired plasticity in T2DM is associated with low glutamatergic metabolite levels. The glutamatergic neurotransmission system constitutes a potential therapeutic target for cognitive problems linked to plasticity-related deficiencies in T2DM.

Proteomic Analysis of Early Diabetic Retinopathy Reveals Mediators of Neurodegenerative Brain Diseases

Purpose

Current evidence suggests that retinal neurodegeneration is an early event in the pathogenesis of diabetic retinopathy. Our main goal was to examine whether, in the diabetic human retina, common proteins and pathways are shared with brain neurodegenerative diseases.

Methods

A proteomic analysis was performed on three groups of postmortem retinas matched by age: nondiabetic control retinas (n = 5), diabetic retinas without glial activation (n = 5), and diabetic retinas with glial activation (n = 5). Retinal lysates from each group were pooled and run on an SDS-PAGE gel. Bands were analyzed sequentially by liquid chromatography-mass spectrometry (LC/MS) using an Orbitrap Mass Spectrometer.

Results

A total of 2190 proteins were identified across all groups. To evaluate the association of the identified proteins with neurological signaling, significant signaling pathways belonging to the category “Neurotransmitters and Other Nervous System Signaling” were selected for analysis. Pathway analysis revealed that “Neuroprotective Role of THOP1 in Alzheimer's Disease” and “Unfolded Protein Response” pathways were uniquely enriched in control retinas. By contrast, “Dopamine Degradation” and “Parkinson's Signaling” were enriched only in diabetic retinas with glial activation. The “Neuregulin Signaling,” “Synaptic Long Term Potentiation,” and “Amyloid Processing” pathways were enriched in diabetic retinas with no glial activation.

Conclusions

Diabetes-induced retinal neurodegeneration and brain neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases, share common pathogenic pathways. These findings suggest that the study of neurodegeneration in the diabetic retina could be useful to further understand the neurodegenerative processes that occur in the brain of persons with diabetes.

The Effect of Agomelatine Treatment on Diabetes-Induced Cognitive Impairments in Rats: Concomitant Alterations in the Hippocampal Neuron Numbers

Researches that are related to the central nervous system complications of diabetes have indicated higher incidence of cognitive disorders in patients. Since the variety of nootropic drugs used in clinics is limited and none of them consistently improves the outcomes, new and effective drug alternatives are needed for the treatment of diabetes-induced cognitive disorders. Based on the nootropic potential of agomelatine, the promising efficacy of this drug on cognitive impairments of diabetic rats was investigated in the current study. Experimental diabetes model was induced by streptozotocin. After development of diabetes-related cognitive impairments in rats, agomelatine (40 and 80 mg/kg) was administrated orally for two weeks. Cognitive performance was assessed by Morris water-maze and passive avoidance tests. Then, the total numbers of neurons in both dentate gyrus and Cornu Ammonis (CA) 1–3 subfields of the hippocampus were estimated by the optical fractionator method. Agomelatine treatment induced notable enhancement in the learning and memory performance of diabetic rats. Moreover, it reversed the neuronal loss in the hippocampal subregions of diabetic animals. Obtained results suggest that agomelatine has a significant potential for the treatment of diabetes-induced cognitive impairments. However, therapeutic efficacy of this drug in diabetic patients suffering from cognitive dysfunctions needs to be confirmed by further clinical trials.

Cognitive dysfunction and metabolic comorbidities in mood disorders: A repurposing opportunity for glucagon-like peptide 1 receptor agonists?

Major depressive disorder and bipolar disorder are highly prevalent and disabling conditions. Cognition is considered a core domain of their psychopathology and a principle mediator of psychosocial impairment, disproportionately accounting for overall illness-associated costs. There are few interventions with replicated evidence of efficacy in treating cognitive deficits in mood disorders. Evidence also indicates that cognitive deficits are associated with obesity and involve significant impairment across multiple domains. Conversely, weight-loss interventions, such as physical exercise and bariatric surgery, have been shown to beneficially affect cognitive function. This convergent phenomenology suggests that currently available agents that target metabolic systems may also be capable of mitigating deficits in cognitive functions, and are, therefore, candidates for repurposing. The incretin glucagon-like peptide-1 (GLP-1) is a hormone secreted by intestinal epithelial cells. GLP-1 receptors (GLP-1R) are widely expressed in the central nervous system. Activation of GLP-1R leads to facilitation of glucose utilization and antiapoptotic effects in various organs. Pre-clinical trials have demonstrated significant neuroprotective effects of GLP-1, including protection from cell death, promotion of neuronal differentiation and proliferation; and facilitation of long-term potentiation. Liraglutide is a GLP-1R agonist that has been approved for the treatment of type 2 diabetes mellitus and obesity. Convergent preclinical and clinical evidence, including a proof-of-concept pilot study from group, has suggested that liraglutide may improve objective measures of cognitive function in adults with mood disorders. The safety and availability of GLP-1R agonists indicate that they are promising candidates for repurposing, and that they may be viable therapeutic options for mood disorders. This article is part of the Special Issue entitled 'Metabolic Impairment as Risk Factors for Neurodegenerative Disorders.'

Changes in Synaptic Plasticity and Glutamate Receptors in Type 2 Diabetic KK-Ay Mice

In the present study, the progressive alteration of cognition and the mechanisms of reduction in long-term potentiation (LTP) in spontaneous obese KK-Ay type 2 diabetic mice were investigated. In the study, 3-, 5-, and 7-month-old KK-Ay mice were used. The results indicated that KK-Ay mice showed cognitive deficits in the Morris water maze test beginning at the age of 3 months. LTP was significantly impaired in KK-Ay mice during whole study period (3 to 7 months). The above deficits were reversible at an early stage (3 to 5 months old) by diet intervention. Moreover, we found the underlying mechanisms of LTP impairment in KK-Ay mice might be attributed to abnormal phosphorylation or expression of postsynaptic glutamate receptor subunits instead of alteration of basal synaptic transmission. The expression levels of NR1, NR2A, and NR2B subunits of N-methyl-d-aspartate receptors (NMDARs) were unchanged while the Tyr-dependent phosphorylation of both NR2A and NR2B subunits were significantly reduced in KK-Ay mice. The level of p-Src expression mediating this process was decreased, and the level of αCaMKII autophosphorylation was also reduced. Meanwhile, the GluR1 of α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs) was decreased, and GluR2 was significantly increased. These data suggest that deficits in synaptic plasticity in KK-Ay mice may arise from the abnormal phosphorylation of the NR2 subunits and the alteration of subunit composition of AMPARs. Diet intervention at an early stage of diabetes might alleviate the cognitive deficits and LTP reduction in KK-Ay mice.

Effects of stevia on synaptic plasticity and NADPH oxidase level of CNS in conditions of metabolic disorders caused by fructose

BACKGROUND

Excess dietary fructose intake associated with metabolic syndrome and insulin resistance and increased risk of developing type 2 diabetes. Previous animal studies have reported that diabetic animals have significantly impaired behavioural and cognitive functions, pathological synaptic function and impaired expression of glutamate receptors. Correction of the antioxidant status of laboratory rodents largely prevents the development of fructose-induced plurimetabolic changes in the nervous system. We suggest a novel concept of efficiency of Stevia leaves for treatment of central diabetic neuropathy.

METHODS

By in vivo extracellular studies induced spike activity of hippocampal neurons during high frequency stimulation of entorhinal cortex, as well as neurons of basolateral amygdala to high-frequency stimulation of the hippocampus effects of Stevia rebaudiana Bertoni plant evaluated in synaptic activity in the brain of fructose-enriched diet rats. In the conditions of metabolic disorders caused by fructose, antioxidant activity of Stevia rebaudiana was assessed by measuring the NOX activity of the hippocampus, amygdala and spinal cord.

RESULTS

In this study, the characteristic features of the metabolic effects of dietary fructose on synaptic plasticity in hippocampal neurons and basolateral amygdala and the state of the NADPH oxidase (NOX) oxidative system of these brain formations are revealed, as well as the prospects for development of multitarget and polyfunctional phytopreparations (with adaptogenic, antioxidant, antidiabetic, nootropic activity) from native raw material of Stevia rebaudiana. Stevia modulates degree of expressiveness of potentiation/depression (approaches but fails to achieve the norm) by shifting the percentage balance in favor of depressor type of responses during high-frequency stimulation, indicating its adaptogenic role in plasticity of neural networks. Under the action of fructose an increase (3-5 times) in specific quantity of total fraction of NOX isoforms isolated from the central nervous system tissue (amygdala, hippocampus, spinal cord) was revealed. Stevia exhibits an antistress, membrane-stabilizing role reducing the level of total fractions of NOX isoforms from central nervous system tissues and regulates NADPH-dependent O2- -producing activity.

CONCLUSION

Generally, in condition of metabolic disorders caused by intensive consumption of dietary fructose Stevia leaves contributes to the control of neuronal synaptic plasticity possibly influencing the conjugated NOX-specific targets.

Treatment with a GLP-1R agonist over four weeks promotes weight loss-moderated changes in frontal-striatal brain structures in individuals with mood disorders

Cognitive deficits are a core feature across psychiatric disorders. Emerging evidence indicates that metabolic pathways are highly relevant for the substrates and phenomenology of the cognitive domain. Herein, we aimed to determine the effects of liraglutide, a GLP-1R agonist, on brain structural/volumetric parameters in adults with a mood disorder. This is the secondary analysis of a 4-week, pilot, proof-of-concept, open-label study. Participants (N=19) exhibiting impairments in executive function with either major depressive disorder (MDD) or bipolar disorder (BD) were recruited. Liraglutide 1.8mg/day was added as an adjunct to existing pharmacotherapy. Structural magnetic resonance imaging (MRI) scanning was obtained at baseline and endpoint. Results showed that at endpoint there was significant weight loss (mean: 3.15%; p<0.001). Changes in frontal and striatal volumes were significantly correlated with changes in body mass index (BMI), indicating the weight loss was associated with volume increase in most regions (e.g. r=-0.561, p=0.042 in the left superior frontal area). After adjusting for intracranial volume, age, gender, and BMI, we observed significant changes from baseline to endpoint in multiple regions (e.g. RR: 1.011, p=0.049 in the left rostral middle frontal area). Changes in regional volumes were associated with improvement in executive function (e.g. r=0.698, p=0.003 for the right superior frontal area). Adjunctive liraglutide results in clinically significant weight loss, with corresponding improvement in cognitive function; changes in cognitive function were partially moderated by changes in brain morphometry, underscoring the interrelationship between weight and brain structure/function.

iTRAQ-based proteomics analysis of hippocampus in spatial memory deficiency rats induced by simulated microgravity

It has been demonstrated that simulated microgravity (SM) may lead to cognitive dysfunction. However, the underlying mechanism remains unclear. In present study, tail-suspension (30°) rat was employed to explore the effects of 28 days of SM on hippocampus-dependent learning and memory capability and the underlying mechanisms. We found that 28-day tail-suspension rats displayed decline of learning and memory ability in Morris water maze (MWM) test. Using iTRAQ-based proteomics analysis, a total of 4774 proteins were quantified in hippocampus. Of these identified proteins, 147 proteins were differentially expressed between tail-suspension and control group. Further analysis showed these differentially expressed proteins (DEPs) involved in different molecular function categories, and participated in many biological processes. Based on the results of PANTHER pathway analysis and further western blot verification, we observed the expression of glutamate receptor 1 (GluR1) and glutamate receptor 4 (GluR4) which involved in metabotropic glutamate receptor group III pathway and ionotropic glutamate receptor pathway were significantly induced by SM. Moreover, an increased concentration of glutamic acid (Glu) was also found in hippocampus while the concentrations of 5-hydroxytryptamine (5-HT), dopamine (DA), γ-amino acid butyric acid (GABA) and epinephrine (E) were decreased. Our finding confirms that 28-day SM exposure can cause degrading of the spatial learning and memory capability and the possible mechanisms might be related with glutamate excitotoxicity and imbalances in specific neurotransmitters.

BIOLOGICAL SIGNIFICANCE

The goal of sending astronauts farther into space and extending the duration of spaceflight missions from months to years will challenge the current capabilities of bioastronautics. The investigation of the physiological and pathological changes induced by spaceflight will be critical in developing countermeasures to ensure astronauts to complete spaceflight mission accurately and effectively and return to earth safely. It has been demonstrated that spaceflight may lead to impairments in cognitive function which is crucial for mission success. Here we show that long-term simulated microgravity, the most potent environment risk factor during spaceflight, impairs the spatial learning and memory of rats and the underlying mechanism may be involved in glutamate excitotoxicity and imbalances in specific neurotransmitters release in hippocampus, which may provide new insight for the countermeasures of cognitive impairment during spaceflight.

Impaired cerebrovascular responsiveness and cognitive performance in adults with type 2 diabetes

AIM

Cognitive deficits in type 2 diabetes mellitus (T2DM) may be partly attributable to stiffness in cerebral arteries and impaired vasodilator function, limiting the ability to increase blood flow in brain regions to meet cognitive demands. We undertook a comparison of cerebrovascular responsiveness (CVR) and cognitive performance in adults with and without T2DM.

METHODS

Older adults with (50) and without (Herath, Cherbuin, Eramudugolla, & Anstey, 2016) T2DM underwent transcranial Doppler ultrasound measurements of basal cerebral mean blood flow velocity (MBFV) and pulsatility index (PI), a measure of arterial stiffness, in the middle cerebral arteries (MCA). A battery of tasks assessing domains of working memory, executive function and information processing/motor speed was then administered while MBFV was recorded. CVR to cognitive tasks was calculated as a percentage increase in MBFV from the basal level.

RESULTS

There was no difference in basal MBFV between groups. However, PI was 14% higher in the T2DM group (P<0.05), who performed poorer across all cognitive domains assessed and displayed poorer CVR in three tasks. Cognitive performance was inversely related to the PI/MBFV ratio, an indicator of intracranial stenosis.

DISCUSSION

Impaired cerebral perfusion during mental tasks is accompanied by poor cognitive performance and stiffness in the cerebral vessels.

Melatonin attenuates the high-fat diet and streptozotocin-induced reduction in rat hippocampal neurogenesis

A deviant level of melatonin in blood circulation has been associated with the development of diabetes and with learning and memory deficiencies. Melatonin might have an important function in diabetes control; however, the mechanism of melatonin in diabetes remains unknown. The present study aimed to investigate the hyperglycemic condition induced by high-fat diet (HFD) feeding and streptozotocin (STZ) injection and to examine the effect of melatonin on adult hippocampal functions. HFD-fed and STZ-treated rats significantly increased blood glucose level. The present study showed that HFD-fed and STZ-treated rats significantly impaired memory in the Morris Water Maze task, reduced neurogenesis in the hippocampus shown by a reduction in nestin, doublecortin (DCX) and β-III tubulin immunoreactivities, reduced axon terminal markers, synaptophysin, reduced dendritic marker including postsynaptic density 95 (PSD-95) and the glutamate receptor subunit NR2A. Moreover, a significant downregulation of melatonin receptor, insulin receptor-β (IR-β) and both p-IR-β and phosphorylated extracellular signal-regulated kinase (p-ERK) occurred in HFD-fed and STZ-treated rats, while the level of glial fibrillary acidic protein (GFAP) increased. Treatment of melatonin, rats had shorter escape latencies and remained in the target quadrant longer compared to the HFD-fed and STZ-treated rats. Melatonin attenuated the reduction of neurogenesis, synaptogenesis and the induction of astrogliosis. Moreover, melatonin countered the reduction of melatonin receptor, insulin receptor and downstream signaling pathway for insulin. Our data suggested that the dysfunction of insulin signaling pathway occurred in the diabetes may provide a convergent mechanism of hippocampal impaired neurogenesis and synaptogenesis lead to impair memory while melatonin reverses these effects, suggesting that melatonin may reduce the pathogenesis of diabetes.

Poor cerebrovascular function is an early marker of cognitive decline in healthy postmenopausal women

INTRODUCTION

Impairment of cerebrovascular function becomes evident after menopause. No study has yet explored relationships between deficits in cerebrovascular function, cognitive performance, and mood in postmenopausal women.

METHOD

Cerebrovascular function was assessed in 80 healthy postmenopausal women by monitoring blood flow velocity (BFV) in the middle and posterior cerebral arteries using transcranial Doppler ultrasound at rest, following a hypercapnic challenge, and during performance of a cognitive test battery; the latter assessed domains of memory and executive functions. Various measures of mood (i.e., Profile of Mood States and Center for Epidemiological Studies Depression Scale) were also assessed.

RESULTS

Cerebral artery elasticity and BFV responsiveness to cognitive tests (neurovascular coupling) correlated with cognitive performance but not with depressive symptoms or mood states. Mood deficits were related to poor cognitive performance.

CONCLUSION

These results highlight the importance of adequate cerebral perfusion for optimized cognitive function in healthy postmenopausal women. Preventative strategies to attenuate accelerated cognitive decline should also consider restoring cerebrovascular function.

Impairment of synaptic development in the hippocampus of diabetic Goto-Kakizaki rats

Insulin receptor signaling has been shown to regulate essential aspects of CNS function such as synaptic plasticity and neuronal survival. To elucidate its roles during CNS development in vivo, we examined the synaptic and cognitive development of the spontaneously diabetic Goto-Kakizaki (GK) rats in the present study. GK rats are non-obese models of type 2 diabetes established by selective inbreeding of Wistar rats based on impaired glucose tolerance. Though they start exhibiting only moderate hyperglycemia without changes in plasma insulin levels from 3 weeks postnatally, behavioral alterations in the open-field as well as significant impairments in memory retention compared with Wistar rats were observed at 10 weeks and were worsened at 20 weeks. Alterations in insulin receptor signaling and signs of insulin resistance were detected in the GK rat hippocampus at 3 weeks, as early as in other insulin-responsive peripheral tissues. Significant reduction of an excitatory postsynaptic scaffold protein, PSD95, was found at 5w and later in the hippocampus of GK rats due to the absence of a two-fold developmental increase of this protein observed in Wistar control rats between 3 and 20w. In the GK rat hippocampus, NR2A which is a NMDA receptor subunit selectively anchored to PSD95 was also reduced. In contrast, both NR2B and its anchoring protein, SAP102, showed similar developmental profiles in Wistar and GK rats with expression peaks at 2 and 3w. The results suggest that early alterations in insulin receptor signaling in the GK rat hippocampus may affect cognitive performance by suppressing synaptic maturation.

Metabonomic profiles delineate potential role of glutamate-glutamine cycle in db/db mice with diabetes-associated cognitive decline

Background

Diabetes-associated cognition decline is one of central nervous system complications in diabetic mellitus, while its pathogenic mechanism remains unclear. In this study, ¹H nuclear magnetic resonance-based metabonomics and immunohistochemistry was used to explore key metabolic alterations in hippocampus of type 2 diabetic db/db mice with cognition decline in order to advance understanding of mechanisms underlying the pathogenesis of the disease.

Results

Metabonomics reveals that lactate level was significantly increased in hippocampus of db/db mice with cognition decline compared with age-matched wild-type mice. Several tricarboxylic acid cycle intermediates including succinate and citrate were reduced in hippocampus of db/db mice with cognition decline. Moreover, an increase in glutamine level and a decrease in glutamate and γ-aminobutyric acid levels were observed in db/db mice. Results from immunohistochemistry analysis show that glutamine synthetase was increased and glutaminase and glutamate decarboxylase were decreased in db/db mice.

Conclusions

Our results suggest that the development of diabetes-associated cognition decline in db/db mice is most likely implicated in a reduction in energy metabolism and a disturbance of glutamate-glutamine shuttling between neurons and astrocytes in hippocampus.

Electronic supplementary material

The online version of this article (doi:10.1186/s13041-016-0223-5) contains supplementary material, which is available to authorized users.

Exenatide enhances cognitive performance and upregulates neurotrophic factor gene expression levels in diabetic mice

Exenatide is a potent and selective agonist for the GLP-1 (glucagon-like peptide-1) receptor. Recent studies are focused on the effects of GLP-1 analogues on hippocampal neurogenesis, cognition, learning and memory functions. The aim of this study was to assess the effects of chronic exenatide treatment (0.1 μg/kg, s.c, twice daily for 2 weeks) on spatial memory functions by using the modified elevated plus maze (mEPM) test and emotional memory functions by using the passive avoidance (PA) test in streptozotocin/nicotinamide (STZ-NA)-induced diabetic mice. As the genes involved in neurite remodelling are among the primary targets of regulation, the effects of diabetes and chronic administration of exenatide on brain-derived neurotrophic factor (BDNF) and cyclic adenosine monophosphate (cAMP) response element binding protein (CREB) messenger ribonucleic acid (mRNA) levels in the hippocampus of mice were also determined using quantitative real-time polymerase chain reaction (RT-PCR). This study revealed that in the mEPM and PA tests, type-2 diabetes-induced mice exhibited significant impairment of learning and memory which were ameliorated by GLP-1 receptor agonist exenatide. Quantitative RT-PCR revealed that CREB and BDNF gene expression levels were downregulated in diabetic mice, and these alterations were increased by exenatide treatment. Since, exenatide improves cognitive ability in STZ/NA-induced diabetic mice and activates molecular mechanisms of memory storage in response to a learning experience, it may be a candidate for alleviation of mood and cognitive disorder.

Glycation in Parkinson's disease and Alzheimer's disease

Glycation is a spontaneous age-dependent posttranslational modification that can impact the structure and function of several proteins. Interestingly, glycation can be detected at the periphery of Lewy bodies in the brain in Parkinson's disease. Moreover, α-synuclein can be glycated, at least under experimental conditions. In Alzheimer's disease, glycation of amyloid β peptide exacerbates its toxicity and contributes to neurodegeneration. Recent studies establish diabetes mellitus as a risk factor for several neurodegenerative disorders, including Parkinson's and Alzheimer's diseases. However, the mechanisms underlying this connection remain unclear. We hypothesize that hyperglycemia might play an important role in the development of these disorders, possibly by also inducing protein glycation and thereby dysfunction, aggregation, and deposition. Here, we explore protein glycation as a common player in Parkinson's and Alzheimer's diseases and propose it may constitute a novel target for the development of strategies for neuroprotective therapeutic interventions. © 2016 International Parkinson and Movement Disorder Society.