Metabolic neuroimaging of the brain in diabetes mellitus and hypoglycaemia

Near-Infrared Spectroscopy: A Free-Living Neuroscience Tool to Better Understand Diabetes and Obesity

The human brain is the least accessible of all organs and attempts to study it in vivo rely predominantly on neuroimaging. Functional near-infrared spectroscopy (fNIRS) allows for the study of cortical neural activity in a non-invasive manner that may resemble free-living conditions. Moreover, compared to other neuroimaging tools, fNIRS is less expensive, it does not require the use of ionizing radiation, and can be applied to all study populations (patients suffering from claustrophobia, or neonates). In this narrative review, we provide an overview of the available research performed using fNIRS in patients with diabetes and obesity. The few studies conducted to date have presented controversial results regarding patients with diabetes, some reporting a greater hemodynamic response and others reporting a reduced hemodynamic response compared to the controls, with an unclear distinction between types 1 and 2. Subjects with obesity or a binge eating disorder have reduced prefrontal activation in response to inhibitory food or non-food stimuli; however, following an intervention, such as cognitive treatment, prefrontal activation is restored. Moreover, we discuss the potential of future applications of fNIRS for a better understanding of cortical neural activity in the context of metabolic disorders.

Glucose metabolic crosstalk and regulation in brain function and diseases

Brain glucose metabolism, including glycolysis, the pentose phosphate pathway, and glycogen turnover, produces ATP for energetic support and provides the precursors for the synthesis of biological macromolecules. Although glucose metabolism in neurons and astrocytes has been extensively studied, the glucose metabolism of microglia and oligodendrocytes, and their interactions with neurons and astrocytes, remain critical to understand brain function. Brain regions with heterogeneous cell composition and cell-type-specific profiles of glucose metabolism suggest that metabolic networks within the brain are complex. Signal transduction proteins including those in the Wnt, GSK-3β, PI3K-AKT, and AMPK pathways are involved in regulating these networks. Additionally, glycolytic enzymes and metabolites, such as hexokinase 2, acetyl-CoA, and enolase 2, are implicated in the modulation of cellular function, microglial activation, glycation, and acetylation of biomolecules. Given these extensive networks, glucose metabolism dysfunction in the whole brain or specific cell types is strongly associated with neurologic pathology including ischemic brain injury and neurodegenerative disorders. This review characterizes the glucose metabolism networks of the brain based on molecular signaling and cellular and regional interactions, and elucidates glucose metabolism-based mechanisms of neurological diseases and therapeutic approaches that may ameliorate metabolic abnormalities in those diseases.

Impaired Retinal Vasoreactivity: An Early Marker of Stroke Risk in Diabetes

Diabetes is a common cause of small vessel disease leading to stroke and vascular dementia. While the function and structure of large cerebral vessels can be easily studied, the brain's microvasculature remains difficult to assess. Previous studies have demonstrated that structural changes in the retinal vessel architecture predict stroke risk, but these changes occur at late disease stages. Our goal was to examine whether retinal vascular status can predict cerebral small vessel dysfunction during early stages of diabetes. Retinal vasoreactivity and cerebral vascular function were measured in 78 subjects (19 healthy controls, 22 subjects with prediabetes, and 37 with type-2 diabetes) using a new noninvasive retinal imaging device (Dynamic Vessel Analyzer) and transcranial Doppler studies, respectively. Cerebral blood vessel responsiveness worsened with disease progression of diabetes. Similarly, retinal vascular reactivity was significantly attenuated in subjects with prediabetes and diabetes compared to healthy controls. Subjects with prediabetes and diabetes with impaired cerebral vasoreactivity showed mainly attenuation of the retinal venous flicker response. This is the first study to explore the relationship between retinal and cerebral vascular function in diabetes. Impairment of venous retinal responsiveness may be one of the earliest markers of vascular dysfunction in diabetes possibly indicating subsequent risk of stroke and vascular dementia.

Brain glucose metabolism during hypoglycemia in type 1 diabetes: insights from functional and metabolic neuroimaging studies

Hypoglycemia is the most frequent complication of insulin therapy in patients with type 1 diabetes. Since the brain is reliant on circulating glucose as its main source of energy, hypoglycemia poses a threat for normal brain function. Paradoxically, although hypoglycemia commonly induces immediate decline in cognitive function, long-lasting changes in brain structure and cognitive function are uncommon in patients with type 1 diabetes. In fact, recurrent hypoglycemia initiates a process of habituation that suppresses hormonal responses to and impairs awareness of subsequent hypoglycemia, which has been attributed to adaptations in the brain. These observations sparked great scientific interest into the brain's handling of glucose during (recurrent) hypoglycemia. Various neuroimaging techniques have been employed to study brain (glucose) metabolism, including PET, fMRI, MRS and ASL. This review discusses what is currently known about cerebral metabolism during hypoglycemia, and how findings obtained by functional and metabolic neuroimaging techniques contributed to this knowledge.

Hypoglycemia-associated changes in the electroencephalogram in patients with type 1 diabetes and normal hypoglycemia awareness or unawareness

Hypoglycemia is associated with increased activity in the low-frequency bands in the electroencephalogram (EEG). We investigated whether hypoglycemia awareness and unawareness are associated with different hypoglycemia-associated EEG changes in patients with type 1 diabetes. Twenty-four patients participated in the study: 10 with normal hypoglycemia awareness and 14 with hypoglycemia unawareness. The patients were studied at normoglycemia (5-6 mmol/L) and hypoglycemia (2.0-2.5 mmol/L), and during recovery (5-6 mmol/L) by hyperinsulinemic glucose clamp. During each 1-h period, EEG, cognitive function, and hypoglycemia symptom scores were recorded, and the counterregulatory hormonal response was measured. Quantitative EEG analysis showed that the absolute amplitude of the θ band and α-θ band up to doubled during hypoglycemia with no difference between the two groups. In the recovery period, the θ amplitude remained increased. Cognitive function declined equally during hypoglycemia in both groups and during recovery reaction time was still prolonged in a subset of tests. The aware group reported higher hypoglycemia symptom scores and had higher epinephrine and cortisol responses compared with the unaware group. In patients with type 1 diabetes, EEG changes and cognitive performance during hypoglycemia are not affected by awareness status during a single insulin-induced episode with hypoglycemia.

Indirect comparison of lixisenatide versus neutral protamine Hagedorn insulin as add-on to metformin and sulphonylurea in patients with type 2 diabetes mellitus

Objective: There is currently a lack of evidence from direct comparisons of treatment outcomes with lixisenatide versus neutral protamine Hagedorn (NPH)-insulin in type 2 diabetes mellitus (T2DM) patients with suboptimal glycaemic control with oral antidiabetic drugs (OADs). Hence, the current analysis indirectly compared available evidence on the risk of hypoglycaemia and weight change between lixisenatide and NPH-insulin based on randomized controlled trial (RCT) data with exenatide, insulin glargine and placebo as common references.

Methods: A systematic search of PubMed, Embase, the Cochrane database and clinical registries identified English- and German-language articles published from January 1980 to October 2012 reporting data from RCTs. Only publications of trials that reported outcomes from 24 to 30 weeks comparing glucagon-like peptide-1 receptor agonists or basal insulin versus another antidiabetic agent or placebo were included. Hypoglycaemia, patients at glycated haemoglobin (HbA_1c) target and discontinuations due to adverse events (AEs) were treated as binary variables, with risk ratios and odds ratios (ORs) calculated. HbA_1c and body weight were treated as continuous variables with difference in mean change from baseline (MD) calculated. Meta-analyses were performed with random effects models and indirect comparisons were performed according to Bucher’s method.

Results: Seven RCTs (n=3,301 patients) comparing the efficacy and safety of lixisenatide, exenatide, insulin glargine and NPH-insulin with different antidiabetic treatments in adult patients with T2DM were included in the final analysis. In the adjusted indirect comparison, there was a significant difference in symptomatic hypoglycaemia (OR = 0.38; 95% CI = [0.17, 0.85]) and in confirmed hypoglycaemia (OR = 0.46; 95% CI = [0.22, 0.96]) favouring lixisenatide over NPH-insulin and comparable changes in HbA_1c from baseline (MD = 0.07%; 95% CI = [–0.26%, 0.41%]). In contrast to NPH-insulin, there was a significant reduction in body weight with lixisenatide (MD = –3.62 kg; 95% CI = [–5.86 kg, –1.38 kg]) at study completion. The number of discontinuations due to AEs numerically favoured NPH-insulin over lixisenatide (OR = 2.64; 95% CI = [0.25, 27.96]), with a broad confidence interval.

Conclusions: Lixisenatide treatment was associated with a lower risk of hypoglycaemia and a greater weight loss compared with NPH-insulin. Glycaemic control with lixisenatide treatment was comparable with NPH-insulin. These data suggest that lixisenatide is a beneficial treatment option for T2DM patients with inadequate glycaemic control on OADs, and is associated with reduced risk of hypoglycaemia and weight gain.

Brain-derived neurotrophic factor inhibits glucose intolerance after cerebral ischemia

Brain-derived neurotrophic factor is associated with the insulin signaling pathway and glucose tabolism. We hypothesized that expression of brain-derived neurotrophic factor and its receptor may be involved in glucose intolerance following ischemic stress. To verify this hypothesis, this study aimed to observe the changes in brain-derived neurotrophic factor and tyrosine kinase B receptor expression in glucose metabolism-associated regions following cerebral ischemic stress in mice. At day 1 after middle cerebral artery occlusion, the expression levels of brain-derived neurotrophic factor were significantly decreased in the ischemic cortex, hypothalamus, liver, skeletal muscle, and pancreas. The expression levels of tyrosine kinase B receptor were decreased in the hypothalamus and liver, and increased in the skeletal muscle and pancreas, but remained unchanged in the cortex. Intrahypothalamic administration of brain-derived neurotrophic factor (40 ng) suppressed the decrease in insulin receptor and tyrosine-phosphorylated insulin receptor expression in the liver and skeletal muscle, and inhibited the overexpression of gluconeogenesis-associated phosphoenolpyruvate carboxykinase and glucose-6-phosphatase in the liver of cerebral ischemic mice. However, serum insulin levels remained unchanged. Our experimental findings indicate that brain-derived neurotrophic factor can promote glucose metabolism, reduce gluconeogenesis, and decrease blood glucose levels after cerebral ischemic stress. The low expression of brain-derived neurotrophic factor following cerebral ischemia may be involved in the development of glucose intolerance.

Banting memorial lecture 2013. A life in balance: wandering the pathways of control

‘To keep in equilibrium’, one of the Oxford English Dictionary’s many definitions of balance, is a desirable target for anylife, but has special meaning for the life of a person with diabetes. Achieving balance—between hypo- and hyperglycaemia; between energy intake and energy consumption; between insulin action and insulin secretion; between attention to diabetes and attention to everything else—remains challenging, but progress has been made over the last three decades, both in our understanding of how nature achieves balance and in the tools we have to try to reproduce the actions of nature in disease states. In particular, the role of the brain in controlling diabetes, from glucose sensing to decision making, has been investigated. Physiological and neuro-imaging studies are finally being translated into patient benefit, with the aim of improving, as Dr Banting put it, the provision of ‘energy for the economic burdens of life’.

Impact of type 1 diabetes mellitus on academic performance

Objective

To examine the effect of type 1 diabetes on academic performance.

Methods

Ethnically Saudi students with type 1 diabetes, and age, sex, ethnicity and socio-economic status-matched nondiabetic control students were recruited from eight schools. Overall academic grades were recorded, based on the scores obtained in written examinations in English, mathematics, physics, chemistry, biology and humanities.

Results

Students with type 1 diabetes ( n = 36) obtained significantly lower academic grades compared with their nondiabetic control classmates ( n = 36) (86.58 ± 1.48 vs 90.62 ± 1.36).

Conclusions

Overall academic performance is significantly lower in students with type 1 diabetes compared with their nondiabetic classmates. This decline in academic performance may be explained by an association between diabetes and cognitive function.