Evaluation of the Effect of Hypoglycemia With PET and a Norepinephrine Transporter Ligand

Overview

The aim of this study is to use Positron Emission Tomography (PET) imaging to measure changes in norepinephrine transporter (NET) concentrations in the brain and periphery of healthy individuals during hypoglycemia. We hypothesize that during hypoglycemia, NE levels will increase within the brain, especially the hypothalamus, and this likely contributes to activation of glucose counterregulatory responses. We further hypothesize that during hypoglycemia, NET concentrations in key glucoregulatory regions will change in order to sustain or prolong sympathetic nervous system activation of counterregulatory responses.

Hypoglycemia elicits a multifaceted hormonal response that aims to restore glycemic levels to normal. As blood glucose levels start to fall, there is a cessation of insulin secretion. At the top of this hierarchy of counterregulatory responses are glucagon and epinephrine, which are the two principal circulating hormones that increase glucose production and inhibit glucose utilization to raise plasma glucose levels back to normal. In conjunction with these circulating hormones there is activation of the sympathetic nervous system, which acts to stimulate hepatic glucose production and lipolysis and suppress peripheral glucose uptake. In cases of prolonged and/or more severe hypoglycemia, growth hormone and cortisol are mobilized to stimulate the synthesis of gluconeogenic enzymes and inhibit glucose utilization.In non-diabetic individuals, glucagon and epinephrine are usually very effective and the latter responses are rarely required in the acute situation. In contrast, impaired glucose counterregulation presents itself in longstanding diabetes and with antecedent hypoglycemia.Within the first five years after the onset of type 1 diabetes, the primary defense against hypoglycemia, the release of glucagon, either becomes significantly attenuated or is completely absent and this impairment appears to be specific for the stimulus of hypoglycemia. Hence, patients with diabetes primarily depend on the release of catecholamines as their main defense against hypoglycemia. Unfortunately, with longer duration of diabetes and especially with poor glycemic control, epinephrine secretion and sympathetic activation are also compromised, making these patients even more vulnerable to the threat of hypoglycemia. In patients with diabetes, hypoglycemia arises from the interplay of a relative excess of exogenous insulin and defective glucose counterregulation and it remains a limiting factor in attaining proper glycemic management. Both the Diabetes Control and Complications Trial (DCCT) conducted in type 1 patients and the United Kingdom Prospective Diabetes Study (UKPDS) conducted in type 2 patients have established the importance of maintaining good glucose control over a lifetime of diabetes to avoid ophthalmologic, renal and neurological complications. However, lowering glycemic goals for patients with diabetes increases their risk for hypoglycemia exposure. According to the DCCT, type 1 patients put on intensive insulin therapy, though having improved outcomes for diabetic complications, are at a 3-fold higher risk of experiencing severe hypoglycemia compared to those on conventional insulin therapy9. Moreover, recent antecedent hypoglycemia reduces autonomic response (catecholamines) and development of symptoms (which normally prompts behavioral defenses such as eating) to subsequent hypoglycemia10-13. Thus begins the vicious cycle of recurrent hypoglycemia where hypoglycemia leads to further impairment of counterregulatory responses which in turn, begets more hypoglycemia and so forth. Because of the imperfections of current insulin therapies, those patients attempting to achieve tight glycemic control suffer an untold number of asymptomatic hypoglycemic episodes. Current estimates of symptomatic hypoglycemic episodes range from 2-3 incidences per week on average and severe, debilitating episodes occur once or twice each year. Therefore, understanding how the body senses falling blood glucose levels and initiates counterregulatory mechanisms will be crucial if we are to prevent or eliminate hypoglycemia. Sensors that detect changes in blood glucose levels and initiate glucose counterregulatory responses have been identified in the hepatic portal vein, the carotid body and most importantly in the brain. In the brain, the predominant sensors are located in the VMH and they are crucial for detecting falling blood glucose levels and for initiating counterregulatory responses. Although the VMH has been implicated as the primary glucose sensor in rodents, no human data are available. Moreover, the exact mechanism leading to VMH activation is not well understood. It was proposed that during hypoglycemia, a rise in VMH norepinephrine (NE) levels improves the counterregulatory response to hypoglycemia27. While these studies highlight the importance of the local NE elevation in the VMH, no one has examined the mechanisms that regulate local NE levels during hypoglycemia. NETs limit the action of NE through reuptake into the cytoplasm, regulating the extent of time that NE remains in the synapse28. Studies in rats showed that chronic elevations of intracerebral insulin can significantly decrease NET mRNA expression in the locus coeruleus, while hypoinsulinemia resulting from streptozotocin-induced diabetes significantly elevates NET mRNA levels. These data suggest that endogenous insulin may be one factor that regulates the synthesis and re-uptake of NE in the CNS. This hypothesis has been confirmed and showed that treating hippocampal tissue and cervical ganglion neurons cells with insulin led to a decrease in NET surface expression. However, the direct effect of insulin on NET levels in humans has never been studied. We have developed a novel approach to measure noradrenergic function using PET scanning and a highly selective norepinephrine transporter (NET) ligand, (S,S)-\[11C\]O-methylreboxetine (\[11C\]MRB). Measuring changes in brain NET concentration is now possible with the use of \[11C\]MRB and a high resolution HRRT PET system.