Orexin glucose-inhibited neurons and hypoglycemia unawareness

Abstract: All individuals with type 1 and >30% of those with type 2 diabetes mellitus (T1/2DM) require intensive insulin therapy to manage their blood glucose levels. This therapy comes at a great price: increased incidence of iatrogenic insulin-induced hypoglycemia. Insulin-induced hypoglycemia is not only acutely life-threatening, repeated hypoglycemia (RH) impairs the ability of the brain to detect and correct subsequent hypoglycemia allowing glucose levels to fall to dangerous, even lethal, levels. Declining glucose first triggers the hormonal (e.g., glucagon, epinephrine) counterregulatory response (CRR) to stimulate gluconeogenesis. Further glucose decline leads to the behavioral response known as hypoglycemia awareness which includes neurogenic (e.g., palpitations, anxiety) and neuroglycopenic (e.g., tiredness, confusion) symptoms that alert the individual of hypoglycemia. RH impairs these hormonal and behavioral responses leading to hypoglycemia associated autonomic failure (HAAF) and hypoglycemia unawareness, respectively. While the CRR and HAAF have been studied extensively, little is known about the mechanisms underlying hypoglycemia awareness/unawareness. This oversight was due to lack of an animal model. However, the Levin lab recently developed a rat model for hypoglycemia awareness which we have translated into the mouse. This model enables us for the first time to investigate how RH causes hypoglycemia unawareness. Our data suggest that orexin expressing neurons in the perifornical hypothalamus (PFH) which are inhibited by glucose (glucose inhibited or GI) play a role in hypoglycemia awareness. We find that RH blunts activation of PFH orexin-GI neuron in low glucose. Using our mouse model of hypoglycemia awareness/unawareness we have shown that the dopamine reuptake inhibitor modafinil restores hypoglycemia awareness after RH. Modafinil also restores glucose sensing by PFH orexin-GI neurons. In this proposal we will test the hypothesis that RH increases the inhibitory effect of glucose on PFH orexin-GI neurons leading to hypoglycemia unawareness. Modafinil, via the dopamine 1 receptor, restores both glucose sensitivity of PFH orexin-GI neurons and hypoglycemia awareness. We will determine how RH alters the glucose sensitivity of PFH orexin-GI neurons and how this is corrected by modafinil. We will also determine the role of dopamine receptors in modafinil’s effect. Finally, we will determine the role of diabetic hyperglycemia per se in hypoglycemia awareness/unawareness. These studies are the first to shed light on the mechanisms of hypoglycemia awareness/unawareness and as such are of critical importance for patients with T1/2DM.