Astrocyte regulation of cerebral blood flow during hypoglycemia

Project Summary Hypoglycemia is a serious complication of diabetes resulting from insulin treatment which can lead to cognitive deficits, brain damage, loss of consciousness, and death. A primary response to hypoglycemia is an increase in cerebral blood flow (CBF). We will test the novel hypothesis that astrocytes contribute to hypoglycemia-induced CBF increases by simultaneously monitoring astrocyte Ca2+ signaling and blood vessel diameter in the mouse cortex as blood glucose is lowered by insulin administration. We will also test whether activity-dependent increases in CBF (neurovascular coupling) is reduced during hypoglycemia and during the course of diabetes. Aim 1. Test the hypothesis that astrocytes mediate hypoglycemia-induced vessel dilation. The relation between blood glucose, astrocyte Ca2+ signaling, and vessel diameter will be determined as blood glucose is lowered by insulin administration in wildtype mice and in IP3R2 KO mice, where astrocyte Ca2+ signaling is reduced. Aim 2. Test the hypothesis that adenosine evokes Ca2+ increases in astrocytes and the release of vasodilating prostaglandins (PGs) and epoxyeicosatrienoic acids (EETs) during hypoglycemia. We will test the hypothesis that adenosine dilation of vessels acts in part by stimulating astrocytes and evoking astrocyte Ca2+ increases. PGs and EETs are released from astrocytes and dilate cerebral vessels. We will test whether one or both of these astrocyte vasodilators contribute to hypoglycemia-induced vessel dilation. Aim 3. Determine whether neurovascular coupling is altered during hypoglycemia. Increases in neuronal activity evoke local increases in CBF. This response, termed neurovascular coupling, supplies active neurons with needed glucose and oxygen. We will test whether vessel dilation evoked by whisker stimulation is altered during hypoglycemia in healthy mice. Aim 4. Test the hypothesis that neurovascular coupling is altered during hypoglycemia in a mouse model of diabetes. Aim 3 experiments will be repeated in diabetic Akita mice. These mice have been subjected to chronic hyperglycemia and represent a more accurate model of insulin-induced hypoglycemia in diabetic patients. Aim 5. Test the hypothesis that neurovascular coupling is altered during the course of long-term hyperglycemia as experienced in the Akita mouse model of diabetes. Cognitive decline is a serious and widespread complication of diabetes and may be caused, in part, by altered CBF regulation. We will determine whether neurovascular coupling is compromised during the course of diabetes in a longitudinal study in the Akita mouse.