The overall goal of this project is to determine the contributions from spiking, synaptic and astrocytic activity in shaping the feature selectivity of blood vessels in the sensory neocortex. This work is important to advance our understanding of brain function because vascular (hemodynamic) signals are now widely used to infer neural function in health and disease. Yet the mechanisms driving many of the spatial and temporal aspects of sensory-evoked hemodynamic signaling are poorly understood. We will perform two-photon functional imaging of neurons, astrocytes and blood vessels in the primary visual cortex of the cat. This animal model shares many sophisticated visual abilities and cortical circuit organizing principles with primates, including spatially precise cortical maps for encoding stimulus orientation. We will use the mapping of stimulus orientation as the probe to determine the contribution of synapses, spikes, and astrocytes in shaping sensory-evoked hemodynamic responses in individual blood vessels. We will combine sub-micron resolution imaging with cell-specific genetically encoded fluorescent sensors for detecting spiking activity (via gCaMP6 imaging) and synaptic activity (via iGluSnFr imaging). We also include artery-specific fluorescent labeling, intracellular recording from astrocytes, and pharmacological blockade of the astrocyte- specific glutamate transporter.
In Aim 1, we test the hypothesis that the selectivit of sensory-evoked dilation of an individual blood vessel is predicted by the spatial pattern of synaptic activity (specifically, glutamate release) immediately surrounding the vessel, not the spatial integration of spiking activity.
In Aim 2 we test the hypothesis that glutamate-driven astrocyte signaling is required for the rapid sensory-evoked activation of blood vessels. To our knowledge, this is the first study in any brain region that will extract sensory-evoked selectivity (tuning curves) from individual blood vessels and relate these single-vessel tuning curves to the local synaptic, spiking and astrocytic activity.
The large size of the mammalian brain makes it difficult for all its regions to receive the maximal blood flow for nourishment and waste removal. The activation of neurons creates a demand for energy that is met by a local and rapid increase in blood flow and this neurovascular coupling can become defective in retinal and neurological diseases including Alzheimer's, Parkinson's, and stroke. This R21 award will support research on the spatial scale and cellular mechanisms of neurovascular coupling in the living brain.