Determining the role of the adrenal Y2 and Y5 receptors in the prevention of hypoglycemia

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PROJECT SUMMARY The objective of the proposed study is to test the hypothesis that Y2 and Y5 receptors, two IDG-eligible proteins play a critical role in determining the adrenal response to hypoglycemia. The significance of this proposal is that in type I diabetes, hypoglycemic episodes are common and disabling. In the diabetic state, the release of insulin is severely reduced and epinephrine becomes the main hormone responsible for restoring euglycemia (the “counter-regulatory response”). However after repeated episodes of low blood glucose, the release of epinephrine also becomes substantially reduced. This phenomenon occurs even in non-diabetic individuals, but why epinephrine release is disrupted is not known. In this proposal we will test the hypothesis that neuropeptide Y, which is a co-transmitter with epinephrine in the adrenal gland, mediates positive (Y5) and negative (Y2) feedback loops that modulate epinephrine release. Increasing the positive (Y5-dependent) or disrupting the negative (Y2-dependent) feedback loops may be a new way to prevent or reverse the consequences of repetitive hypoglycemia. To validate the role of the Y2 and Y5 receptors in this pathway we have two specific aims. First, using BAC transgenic reporter mice and calcium imaging we will determine which adrenal cells express Y2 and Y5 receptors. The adrenal medulla contains a variety of cell types. We will investigate whether the site of receptor expression involves non-neuronal cells, in particular a population of macrophages implicated in the control of adrenal function. Second, we will test the hypothesis that the adrenal Y2 and Y5 receptors are core components of two interacting feedback loops that modulate blood glucose levels during hypoglycemia. We will determine whether NPY release from chromaffin cells leads to a rapid Y5-dependent increase in preganglionic → chromaffin cell synaptic strength and a delayed Y2-dependent suppression of adrenal NPY synthesis. Finally, we will test whether selective blockade of Y2- and Y5R’s in vivo can temporally separate these feedback loops. The involvement of peripheral Y2 and Y5R’s in the homeostatic control of blood glucose levels represents a new role for these proteins and confirming their involvement would strengthen the rationale for future work investigating the clinical utility of modulating this signaling pathway.