Hypoglycemia-associated autonomic failure, or HAAF, is a syndrome associated with type1 and type2 diabetes that results from the experience of multiple episodes of low blood sugar (hypoglycemia) within a short timeframe. It is characterized by impairment of both the sensing of symptoms of falling blood glucose and the neuroendocrine counterregulatory response (CRR), a hormonal and neural response that includes sympathoadrenal activation and epinephrine (EPI) release; activation of the hypothalamic/pituitary/adrenal (HPA) axis and release of adrenal glucocorticoids; and glucagon (GLU) release. Most critical of these responses are the GLU and EPI release, and consequent stimulation of hepatic glucose release within minutes. In the diabetic individual, the GLU response is lost early in the disease state, and thus diabetic individuals are critically dependent upon EPI release for the CRR to hypoglycemia. For the past ten years, we have been studying brain sites that play a critical role in the mediation of the CRR in a (non-diabetic) rodent model of HAAF. Our lab and others have identified neural cell groups in the brainstem, medial hypothalamus, and limbic forebrain that are linked to each other and contribute to the CRR. However, there is very little information available about the activation or response of the diabetic brain to a hypoglycemic challenge. We now propose to systematically evaluate the key neural elements of the CRR in a model of acute diabetes, with the long-term objective of determining the components of impaired responding as potential targets for therapeutic intervention. Using a streptozotocin (STZ)-diabetic rat model and hyperinsulinemic hypoglycemic clamp methodology, we will measure neural activation (cFos, pCREB, and pERK1/2 expression) in response to a single (SH) or a third (recurrent, RH) bout of hypoglycemia throughout the brain. We will also screen the spinal cord and adrenal gland, i.e., the pathway of activation of the adrenal gland from the brain. We will determine the responsivity of brain glucose-sensing sites to a localized neuroglucopenic stimulus. Finally, we will evaluate the effect of the serotonergic agent sertraline (SERT) on the hypoglycemia CRR in diabetic rats, as we have observed enhanced EPI responding in non-diabetic rats treated with this agent. The proposed studies will thus achieve the short-term overall objective of evaluating the sensing of, and response to, neuroglucopenia and hypoglycemia in an animal model of diabetes.
The prevalence and incidence of both type1 and type2 diabetes in the U.S., other 'Westernized', and emerging countries is high, and has been increasing in the U.S. for the past decade, with current CDC estimates of 7.5 individuals per thousand in the population overall. Further, both type1 and type2 diabetes are prevalent at a high rate in the veteran population: Currently, it is estimated that 16% of men receiving medical care at the VA have diabetes. Diabetes is a costly disease from every perspective, including medical costs for complications, and psychological distress for both the patient and the care-givers in the patient's life. Care of patients in which poor blood sugar control has resulted in one or more of the chronic complications of diabetes is costly and time- intensive in terms of VA staff support and VA services. The goal of intensified insulin therapy to delay, prevent, or decrease the severity, of long-term complications is still, theoretically, the best treatment regimen available. However, the recent report of increased mortality, with no identified cardiovascular benefit, in a population of intensively-treated diabetic individuals highlights the need to address hypoglycemia as a diabetic complication. The response to hypoglycemia (and the failed response which occurs with frequent episodes of hypoglycemia) is clearly rooted in altered brain function, thus understanding the brain mechanisms underlying the response to hypoglycemia in diabetic animals or humans is critical. We have studied the activity of key brain regions in response to hypoglycemia in a non-diabetic animal model. We are now proposing to study brain activation in an animal model of diabetes, with the rationale that identifying brain regions that are vulnerable to hypoglycemia, as well as identification of their role in an impaired biochemical response to hypoglycemia, is the first critical step to ancillary therapies to prevent hypoglycemia in association with intensive insulin therapy in diabetes.
