Orexin signaling is necessary for hypoglycemia-induced prevention of conditioned place preference

Still Excited, but Less Aroused-The Effects of Nutritional Ketosis on Epinephrine Response and Hypothalamic Orexin Neuron Activation Following Recurrent Hypoglycemia in Diabetic Rats

Hypoglycemia-associated autonomic failure (HAAF) is a serious, life-threatening complication of intensive insulin therapy, particularly in people with type 1 diabetes. The ketogenic diet is reported to beneficially affect glycemic control in people with type 1 diabetes, however its effects on the neurohormonal counterregulatory response to recurrent hypoglycemia and HAAF development are understudied. In this study we used Sprague Dawley rats to establish a HAAF model under non-diabetic and streptozotocin (STZ)-induced diabetic conditions and determined how nutritional ketosis affected the neurohormonal counterregulation and the activity of energy-sensing orexin (OX) neurons. We found that antecedent hypoglycemia diminished the sympathoexcitatory epinephrine response to subsequent hypoglycemia in chow-fed non-diabetic rats, but this did not occur in STZ-diabetic animals. In all cases a ketogenic diet preserved the epinephrine response. Contrary to expectations, STZ-diabetic keto-fed rats showed reduced OX activity in the recurrent hypoglycemia group, which did not occur in any other group. It is possible that the reduced activation of OX neurons is an adaptation aimed at energy conservation accompanied by diminished arousal and exploratory behaviour. Our data suggests that while a ketogenic diet has beneficial effects on glycemia, and epinephrine response, the reduced activation of OX neurons could be detrimental and warrants further investigation.

Impaired Awareness of Hypoglycemia in Type 1 Diabetes: A Report of An NIDDK Workshop in October 2021

Hypoglycemia remains a limiting factor in the optimal treatment of type 1 diabetes. Repeated episodes of hypoglycemia result in impaired awareness of subsequent hypoglycemic events, inducing a vicious feed-forward cycle and increasing the risk of morbidity and mortality. Why this occurs and how to manage the problem in clinical practice remain uncertain. To address the obstacles and barriers that have hindered progress in this clinically important area, the National Institute of Diabetes and Digestive and Kidney Diseases convened a workshop on 14-15 October 2021. This perspective offers a summary of this outstanding meeting, which brought clinical and basic scientists from the fields of diabetes, neuroscience, psychology, psychiatry, and imaging together, on how to best advance the field of impaired awareness of hypoglycemia and hypoglycemia in general in patients with diabetes.

Insulin actions on hypothalamic glucose-sensing neurones

Subsequent to the discovery of insulin 100 years ago, great strides have been made in understanding its function, especially in the brain. It is now clear that insulin is a critical regulator of the neuronal circuitry controlling energy balance and glucose homeostasis. This review focuses on the effects of insulin and diabetes on the activity and glucose sensitivity of hypothalamic glucose-sensing neurones. We highlight the role of electrophysiological data in understanding how insulin regulates glucose-sensing neurones. A brief introduction describing the benefits and limitations of the major electrophysiological techniques used to investigate glucose-sensing neurones is provided. The mechanisms by which hypothalamic neurones sense glucose are discussed with an emphasis on those glucose-sensing neurones already shown to be modulated by insulin. Next, the literature pertaining to how insulin alters the activity and glucose sensitivity of these hypothalamic glucose-sensing neurones is described. In addition, the effects of impaired insulin signalling during diabetes and the ramifications of insulin-induced hypoglycaemia on hypothalamic glucose-sensing neurones are covered. To the extent that it is known, we present hypotheses concerning the mechanisms underlying the effects of these insulin-related pathologies. To conclude, electrophysiological data from the hippocampus are evaluated aiming to provide clues regarding how insulin might influence neuronal plasticity in glucose-sensing neurones. Although much has been accomplished subsequent to the discovery of insulin, the work described in our review suggests that the regulation of central glucose sensing by this hormone is both important and understudied.

Brain-Body Control of Glucose Homeostasis-Insights From Model Organisms

Tight regulation of blood glucose is essential for long term health. Blood glucose levels are defended by the correct function of, and communication between, internal organs including the gastrointestinal tract, pancreas, liver, and brain. Critically, the brain is sensitive to acute changes in blood glucose level and can modulate peripheral processes to defend against these deviations. In this mini-review we highlight select key findings showcasing the utility, strengths, and limitations of model organisms to study brain-body interactions that sense and control blood glucose levels. First, we discuss the large platform of genetic tools available to investigators studying mice and how this field may yet reveal new modes of communication between peripheral organs and the brain. Second, we discuss how rats, by virtue of their size, have unique advantages for the study of CNS control of glucose homeostasis and note that they may more closely model some aspects of human (patho)physiology. Third, we discuss the nascent field of studying the CNS control of blood glucose in the zebrafish which permits ease of genetic modification, large-scale measurements of neural activity and live imaging in addition to high-throughput screening. Finally, we briefly discuss glucose homeostasis in drosophila, which have a distinct physiology and glucoregulatory systems to vertebrates.

Experimental Models of Impaired Hypoglycaemia-Associated Counter-Regulation

Impaired awareness of hypoglycaemia (IAH) affects around a quarter of patients with diabetes who receive insulin treatment. This condition is characterised by a progressive reduction in symptomatic and behavioural responses to hypoglycaemia, increasing risk of deeper drops in blood glucose, unconsciousness, and collapse. Thus, patients with IAH experience severe hypoglycaemic episodes more frequently, resulting in significant morbidity and mortality. IAH is thought to develop as a consequence of whole-body adaptations to repeated insulin-induced hypoglycaemia (RH), with widespread deficits in the hypoglycaemia counter-regulatory response (CRR). Despite this important insight, the precise pathophysiology by which RH leads to an attenuated CRR is unknown. Studies into the underlying mechanisms of IAH have employed a variety of protocols in humans and experimental species. The use of animal models has many investigational benefits, including the unprecedented increase in the availability of transgenic strains. However, modelling impaired hypoglycaemia-associated counter-regulation remains challenging and appropriate interpretation of findings across species and protocols even more so. Here, we review the experimental modelling of IAH and impaired hypoglycaemia-associated counter-regulation, with a focus on understanding species-specific variation in glucose homeostasis. This review will aid investigators in interpreting outputs from different studies in IAH and aid progress in the field.

Ventromedial hypothalamus glucose-inhibited neurones: A role in glucose and energy homeostasis?

The ventromedial hypothalamus (VMH) plays a complex role in glucose and energy homeostasis. The VMH is necessary for the counter-regulatory response to hypoglycaemia (CRR) that increases hepatic gluconeogenesis to restore euglycaemia. On the other hand, the VMH also restrains hepatic glucose production during euglycaemia and stimulates peripheral glucose uptake. The VMH is also important for the ability of oestrogen to increase energy expenditure. This latter function is mediated by VMH modulation of the lateral/perifornical hypothalamic area (lateral/perifornical hypothalamus) orexin neurones. Activation of VMH AMP-activated protein kinase (AMPK) is necessary for the CRR. By contrast, VMH AMPK inhibition favours decreased basal glucose levels and is required for oestrogen to increase energy expenditure. Specialised VMH glucose-sensing neurones confer the ability to sense and respond to changes in blood glucose levels. Glucose-excited (GE) neurones increase and glucose-inhibited (GI) neurones decrease their activity as glucose levels rise. VMH GI neurones, in particular, appear to be important in the CRR, although a role for GE neurones cannot be discounted. AMPK mediates glucose sensing in VMH GI neurones suggesting that, although activation of these neurones is important for the CRR, it is necessary to silence them to lower basal glucose levels and enable oestrogen to increase energy expenditure. In support of this, we found that oestrogen reduces activation of VMH GI neurones in low glucose by inhibiting AMPK. In this review, we present the evidence underlying the role of the VMH in glucose and energy homeostasis. We then discuss the role of VMH glucose-sensing neurones in mediating these effects, with a strong emphasis on oestrogenic regulation of glucose sensing and how this may affect glucose and energy homeostasis.

Hypoglycemia: Role of Hypothalamic Glucose-Inhibited (GI) Neurons in Detection and Correction

Hypoglycemia is a profound threat to the brain since glucose is its primary fuel. As a result, glucose sensors are widely located in the central nervous system and periphery. In this perspective we will focus on the role of hypothalamic glucose-inhibited (GI) neurons in sensing and correcting hypoglycemia. In particular, we will discuss GI neurons in the ventromedial hypothalamus (VMH) which express neuronal nitric oxide synthase (nNOS) and in the perifornical hypothalamus (PFH) which express orexin. The ability of VMH nNOS-GI neurons to depolarize in low glucose closely parallels the hormonal response to hypoglycemia which stimulates gluconeogenesis. We have found that nitric oxide (NO) production in low glucose is dependent on oxidative status. In this perspective we will discuss the potential relevance of our work showing that enhancing the glutathione antioxidant system prevents hypoglycemia associated autonomic failure (HAAF) in non-diabetic rats whereas VMH overexpression of the thioredoxin antioxidant system restores hypoglycemia counterregulation in rats with type 1 diabetes.We will also address the potential role of the orexin-GI neurons in the arousal response needed for hypoglycemia awareness which leads to behavioral correction (e.g., food intake, glucose administration). The potential relationship between the hypothalamic sensors and the neurocircuitry in the hindbrain and portal mesenteric vein which is critical for hypoglycemia correction will then be discussed.

PACAP Neurons in the Ventromedial Hypothalamic Nucleus Are Glucose Inhibited and Their Selective Activation Induces Hyperglycaemia

Background: Glucose-sensing neurons are located in several parts of the brain, but are concentrated in the ventromedial nucleus of the hypothalamus (VMH). The importance of these VMH neurons in glucose homeostasis is well-established, however, little is known about their individual identity. In the present study, we identified a distinct glucose-sensing population in the VMH and explored its place in the glucose-regulatory network. Methods: Using patch-clamp electrophysiology on Pacap-cre::EYFP cells, we explored the glucose-sensing ability of the pituitary adenylate cyclase-activating peptide (PACAP) neurons both inside and outside the VMH. We also mapped the efferent projections of these neurons using anterograde and retrograde tracing techniques. Finally, to test the functionality of PACAP^VMH in vivo, we used DREADD technology and measured systemic responses. Results: We demonstrate that PACAP neurons inside (PACAP^VMH), but not outside the VMH are intrinsically glucose inhibited (GI). Anatomical tracing techniques show that PACAP^VMH neurons project to several areas that can influence autonomic output. In vivo, chemogenetic stimulation of these neurons inhibits insulin secretion leading to reduced glucose tolerance, implicating their role in systemic glucose regulation. Conclusion: These findings are important as they identify, for the first time, a specific VMH neuronal population involved in glucose homeostasis. Identifying the different glucose-sensing populations in the VMH will help piece together the different arms of glucose regulation providing vital information regarding central responses to glucose metabolic disorders including hypoglycaemia.

10 lessons learned by a misguided physician

It was a great and humbling honor to receive the 2016 Distinguished Career Award from my SSIB colleagues. This paper summarizes the major points of my DCA talk at the 2016 annual meeting. It is a reflection on my 50year medical and research career and 10 lessons I have learned over those years which might be of help to young investigators near the beginning of their own research careers. These lessons include: the value of being receptive to the opportunities provided you; how clinician-scientists can serve as critical role models for young investigators like me and a history of how my career developed as a result of their influence; the importance of carefully examining your own data, particularly when it doesn't agree with your preconceived ideas; the critical role that students, postdocs and PhD (and even veterinarian) colleagues can play in developing one's career; the likelihood that your career path will have many interesting twists and turns determined by changes in your own scientific interests and how rewarding various areas of research focus are to you; the importance of building a close-knit laboratory staff family; the fact that science and romance can mix. Finally, I offer 3 somewhat self-evident free pieces of advice for building and maintaining a rewarding career.

Targeting orphan G protein-coupled receptors for the treatment of diabetes and its complications: C-peptide and GPR146

G protein-coupled receptors (GPCRs) are the most abundant receptor family encoded by the human genome and are the targets of a high percentage of drugs currently in use or in clinical trials for the treatment of diseases such as diabetes and its associated complications. Thus, orphan GPCRs, for which the ligand is unknown, represent an important untapped source of therapeutic potential for the treatment of many diseases. We have identified the previously orphan GPCR, GPR146, as the putative receptor of proinsulin C-peptide, which may prove to be an effective treatment for diabetes-associated complications. For example, we have found a potential role of C-peptide and GPR146 in regulating the function of the retinal pigment epithelium, a monolayer of cells in the retina that serves as part of the blood-retinal barrier and is disrupted in diabetic macular oedema. However, C-peptide signalling in this cell type appears to depend at least in part on extracellular glucose concentration and its interaction with insulin. In this review, we discuss the therapeutic potential of orphan GPCRs with a special focus on C-peptide and GPR146, including past and current strategies used to 'deorphanize' this diverse family of receptors, past successes and the inherent difficulties of this process.

Hypoglycemia unawareness prevention: Targeting glucagon production

Insulin-dependent individuals with diabetes are at risk for a severe hypoglycemic event that may predispose them to several repeat episodes during which the normal counter regulatory mechanisms that protect against hypoglycemia fail to be activated. This state of hypoglycemia unawareness is characterized by a failure of glucagon release, preventing mobilization of endogenous glucose stores from the liver. We describe the discovery of a novel hormone, produced in pancreatic delta cells, which stimulates glucagon production and release, particularly under low glucose conditions. We hypothesize that this hormone, called neuronostatin, may be effective as a co-therapy with insulin to prevent repeated, potentially fatal episodes of recurrent hypoglycemia.