|Vanessa H. Routh, PhD, associate professor, Department of Pharmacology & Physiology, UMDNJ-New Jersey Medical School|
Our laboratory studies the function of specialized glucose sensing neurons, which may enable the brain to sense and respond to the metabolic status of the body. We have shown that the glucose sensitivity of these neurons follows changes in peripheral energy status in both health and diabetes. Our long-term goal is to understand the cellular mechanisms allowing these neurons to sense glucose and respond to the metabolic state of the body in health, obesity and diabetes.
We have only to read the news or look around us to know that obesity and Type 2 diabetes mellitus are serious health issues. In fact, obesity and its associated co-morbidities (e.g., heart disease, hypertension, stroke, and cancer) are the second leading cause of death in the United States. Poor dietary habits and a sedentary lifestyle contribute to this epidemic. However, it is also clear that obesity and Type 2 diabetes mellitus are preceded by an underlying dysfunction in the regulation of energy homeostasis. One of the earliest indications of disease is peripheral insulin resistance combined with impaired insulin secretion in response to glucose. While most people probably do not suspect that their brain is at fault for their increasing waistline or diabetes, there is no doubt that the brain plays a major role in regulating energy homeostasis. Moreover, growing evidence indicates that dysfunction in the way that the brain senses nutrients contributes strongly to the development of peripheral insulin resistance. However, the mechanism wherein the brain actually senses and responds to the metabolic status of the body remains a mystery.
Glucose regulates specialized neurons. These “glucose sensing neurons” reside in a number of brain regions associated with energy homeostasis, including the ventromedial hypothalamus (VMH). We hypothesize that VMH glucose sensing neurons enable the brain to sense and respond to the body’s metabolic status. Moreover, aberration of these neurons impairs the brain’s regulation of energy homeostasis. Dr. Zhentao Song, a member of our research team, and I were the first to characterize glucose sensing neurons using glucose levels which would be seen in the living brain during normal daily fluctuations, as well as during diabetes. In the brain of a fed rat, glucose concentration is ~1.5 mM. After the equivalent of a day-long fast in humans, glucose concentration in the rat brain is 0.7 mM. During severe uncompensated diabetes, where glucose concentrations in the blood can reach 20 mM (360 mg/dl), brain glucose concentrations are 4.5 mM. There are two types of glucose sensing neurons which respond directly to changes in extracellular glucose concentration within this range. Glucose-inhibited (GI) neurons decrease, while glucose-excited (GE) neurons increase their activity as extracellular glucose levels increase. Their response to glucose is steeply linear from 0.5 to 1.5 mM, after which it begins to plateau, showing no further response above 5 mM. Therefore, GI and GE neurons are exquisitely sensitive to glucose concentrations seen in the brain under normal physiological conditions. Since glucose sensing neurons were originally studied in 10 or 20 mM glucose, it was not until we established that they respond to normal brain glucose levels that they could be seriously considered as mediators of the brain’s response to metabolic status.
|A Control||B Diabetic|
| Figure 1. Fluorescence of the nitric oxide (NO)-sensitive dye, diaminofluorescein
(DAF-FM) in glucose-inhibited (GI) neurons from the ventromedial hypothalamus (VMH). DAF-FM fluorescence increases when NO is produced. The presence of DAF-FM fluorescence in control rats indicates that VMH GI neurons produce nitric oxide under steady state conditions. DAF-FM fluorescence (NO production) increases in these neurons as extracellular glucose concentration decreases. In contrast, VMH GI neurons from diabetic rats do not produce nitric oxide under steady state conditions or in response to decreased glucose. Canabal et al., AJP 2007.
GI neurons sense glucose by AMP-activated protein kinase (AMPK) mediated regulation of a chloride channel (the cystic fibrosis transmembrane regulator, CFTR). Dr. Debra Canabal, a recent graduate from our laboratory who is now at Research Diets, showed that the gaseous messenger, nitric oxide (NO), is also involved in the glucose response of GI neurons. Like pancreatic beta cells, the effects of glucose in GE neurons are transduced by the ATP-sensitive potassium (KATP) channel. Victoria Cotero, a PhD candidate in our laboratory, discovered that insulin regulates the glucose sensitivity of GE neurons.
The glucose sensitivity of VMH glucose sensing neurons is related to the metabolic status of the body. For example, Beth Murphy, a PhD candidate in our laboratory, recently discovered that the glucose sensitivity of VMH GI neurons differs in the fed and fasted state; in fed mice glucose concentrations must decrease further to elicit the same response seen in GI neurons from fasted mice. This suggests that VMH GI neurons respond more strongly to decreased blood glucose when peripheral energy stores are depleted by a fast. Interestingly, when VMH GI neurons are exposed to “diabetic” levels of brain glucose (5 mM), they no longer alter their activity and NO production in response to glucose or insulin. Furthermore, NO production in VMH GI neurons is absent in rats with Type 1 diabetes mellitus (Figure 1). Both changes in GI neurons’ activity and NO production are restored when AMPK is re-activated. Alterations in GI and GE neurons may also lead to the development of Type 2 diabetes mellitus. We have shown that VMH GI and GE neurons, as well as VMH KATP channels are dysfunctional in pre-diabetic rats that Dr. Barry Levin (see next article) selectively bred for the genetic tendency to develop obesity and Type 2 diabetes mellitus after eating a diet frighteningly similar to the standard American diet.
Finally, hypoglycemia, the major limiting factor in intensive insulin therapy used for Type 1 and advanced Type 2 diabetes mellitus, prevents the brain from detecting future hypoglycemic episodes and generating protective responses that restore blood glucose levels to normal. Dr. Song’s work revealed that VMH GI and GE neurons from rats subjected to insulin-hypoglycemia are less sensitive to decreased glucose. Impaired NO production in GI neurons may be responsible. Dr. Xavier Fioramonti, a postdoctoral fellow in our laboratory, discovered that blocking VMH NO production also prevented the brain from generating the full protective response to hypoglycemia.
Thus, our data strongly suggest that VMH GI and GE neurons enable the brain to sense and respond to changes in metabolic status. Moreover, glucose sensitivity of VMH GI and GE neurons is reduced by diabetic hyperglycemia and insulin-induced hypoglycemia. Since the brain does not detect and respond normally to hypoglycemia under these conditions, it suggests that dysfunction in VMH GI and GE neurons underlies impaired brain glucose sensing. Finally, dysfunctional glucose sensing by VMH GI and GE neurons may play a role in the development of obesity and Type 2 diabetes mellitus. It is our long-term goal to understand the cellular mechanisms by which glucose sensing neurons sense glucose and how these mechanisms become dysfunctional during obesity and diabetes.
Vanessa Routh earned her PhD from the Department of Animal Physiology at the University of California at Davis, and received postdoctoral training at the Uniformed Services University of the Health Sciences (MD) and at UMDNJ. She joined the faculty of the Department of Pharmacology & Physiology at UMDNJ-New Jersey Medical School in 1998 and is currently an associate professor there.