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), which augments the supply of glucose to the brain. A hypoglycemia-induced increase in adenosine in the brain is thought to mediate CBF increases. However, astrocytes, which release vasodilating agents and regulate vascular tone, might also contribute to hypoglycemia-induced vessel dilation and CBF increases. This novel hypothesis, that astrocytes contribute to hypoglycemia-induced CBF increases, is supported by our preliminary experiments. We will test this hypothesis by simultaneously monitoring astrocyte Ca2+ signaling and blood vessel diameter in the somatosensory cortex of awake mice with two-photon microscopy as blood glucose is lowered by insulin administration. The hypothesis will be tested in the following aims. 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 control mice and in IP3R2 KO mice, where astrocyte Ca2+ signaling is reduced. As suggested by our preliminary results, we anticipate that vessel dilation will be reduced in IP3R2 KO animals, demonstrating that astrocytes contribute to hypoglycemia-induced vessel dilation. 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. Adenosine mediates hypoglycemia-induced CBF increases. 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 functional hyperemia, supplies active neurons with needed glucose and oxygen. We will test whether vessel dilation evoked by whisker stimulation is altered during hypoglycemia.